Add missing docs for System.Collections.Immutable (#4060)

* Add missing docs for System.Collections.Immutable

* Update xml/System.Collections.Immutable/ImmutableArray.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1+Builder.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableDictionary`2+Builder.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* Update xml/System.Collections.Immutable/ImmutableArray`1.xml

Co-Authored-By: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

* address feedback on remarks

* address more missing remarks

Co-authored-by: Carlos Sanchez Lopez <1175054+carlossanlop@users.noreply.github.com>

Author: eiriktsarpalis

xml/System.Collections.Immutable/ImmutableArray.xml

@@ -679,12 +679,12 @@ This overload allows helper methods or custom builder classes to efficiently avo
         <Parameter Name="selector" Type="System.Func&lt;TSource,TResult&gt;" />
       </Parameters>
       <Docs>
-        <typeparam name="TSource">To be added.</typeparam>
-        <typeparam name="TResult">To be added.</typeparam>
+        <typeparam name="TSource">The type of element stored in the source array.</typeparam>
+        <typeparam name="TResult">The type of element to store in the target array.</typeparam>
         <param name="items">The source array to initialize the resulting array with.</param>
         <param name="selector">The function to apply to each element from the source array.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array that contains the specified items.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -728,14 +728,14 @@ This overload allows helper methods or custom builder classes to efficiently avo
         <Parameter Name="selector" Type="System.Func&lt;TSource,TResult&gt;" />
       </Parameters>
       <Docs>
-        <typeparam name="TSource">To be added.</typeparam>
-        <typeparam name="TResult">To be added.</typeparam>
+        <typeparam name="TSource">The type of element stored in the source array.</typeparam>
+        <typeparam name="TResult">The type of element to store in the target array.</typeparam>
         <param name="items">The source array to initialize the resulting array with.</param>
         <param name="start">The index of the first element in the source array to include in the resulting array.</param>
         <param name="length">The number of elements from the source array to include in the resulting array.</param>
         <param name="selector">The function to apply to each element from the source array included in the resulting array.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array that contains the specified items.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -779,14 +779,14 @@ This overload allows helper methods or custom builder classes to efficiently avo
         <Parameter Name="arg" Type="TArg" />
       </Parameters>
       <Docs>
-        <typeparam name="TSource">To be added.</typeparam>
-        <typeparam name="TArg">To be added.</typeparam>
-        <typeparam name="TResult">To be added.</typeparam>
+        <typeparam name="TSource">The type of element stored in the source array.</typeparam>
+        <typeparam name="TArg">The type of argument to pass to the selector mapping function.</typeparam>
+        <typeparam name="TResult">The type of element to store in the target array.</typeparam>
         <param name="items">The source array to initialize the resulting array with.</param>
         <param name="selector">The function to apply to each element from the source array.</param>
         <param name="arg">An argument to be passed to the selector mapping function.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array that contains the specified items.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -832,16 +832,16 @@ This overload allows helper methods or custom builder classes to efficiently avo
         <Parameter Name="arg" Type="TArg" />
       </Parameters>
       <Docs>
-        <typeparam name="TSource">To be added.</typeparam>
-        <typeparam name="TArg">To be added.</typeparam>
-        <typeparam name="TResult">To be added.</typeparam>
+        <typeparam name="TSource">The type of element stored in the source array.</typeparam>
+        <typeparam name="TArg">The type of argument to be passed to the selector mapping function.</typeparam>
+        <typeparam name="TResult">The type of element to be stored in the target array.</typeparam>
         <param name="items">The source array to initialize the resulting array with.</param>
         <param name="start">The index of the first element in the source array to include in the resulting array.</param>
         <param name="length">The number of elements from the source array to include in the resulting array.</param>
         <param name="selector">The function to apply to each element from the source array included in the resulting array.</param>
         <param name="arg">An argument to be passed to the selector mapping function.</param>
         <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array that contains the specified items.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   

xml/System.Collections.Immutable/ImmutableArray`1+Builder.xml

@@ -1,4 +1,4 @@
-<Type Name="ImmutableArray&lt;T&gt;+Builder" FullName="System.Collections.Immutable.ImmutableArray&lt;T&gt;+Builder">
+<Type Name="ImmutableArray&lt;T&gt;+Builder" FullName="System.Collections.Immutable.ImmutableArray&lt;T&gt;+Builder">
   <TypeSignature Language="C#" Value="public sealed class ImmutableArray&lt;T&gt;.Builder : System.Collections.Generic.ICollection&lt;T&gt;, System.Collections.Generic.IEnumerable&lt;T&gt;, System.Collections.Generic.IList&lt;T&gt;, System.Collections.Generic.IReadOnlyCollection&lt;T&gt;, System.Collections.Generic.IReadOnlyList&lt;T&gt;" />
   <TypeSignature Language="ILAsm" Value=".class nested public auto ansi sealed beforefieldinit ImmutableArray`1/Builder&lt;T&gt; extends System.Object implements class System.Collections.Generic.ICollection`1&lt;!T&gt;, class System.Collections.Generic.IEnumerable`1&lt;!T&gt;, class System.Collections.Generic.IList`1&lt;!T&gt;, class System.Collections.Generic.IReadOnlyCollection`1&lt;!T&gt;, class System.Collections.Generic.IReadOnlyList`1&lt;!T&gt;, class System.Collections.IEnumerable" />
   <TypeSignature Language="DocId" Value="T:System.Collections.Immutable.ImmutableArray`1.Builder" />
@@ -1029,7 +1029,7 @@
       <Parameters />
       <Docs>
         <summary>Extracts the internal array as an <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> and replaces it              with a zero length array.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array containing the elements of the builder.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidOperationException">When <see cref="P:System.Collections.Immutable.ImmutableArray`1.Builder.Count" /> doesn't              equal <see cref="P:System.Collections.Immutable.ImmutableArray`1.Builder.Capacity" />.</exception>
       </Docs>

xml/System.Collections.Immutable/ImmutableArray`1.xml

@@ -1,4 +1,4 @@
-<Type Name="ImmutableArray&lt;T&gt;" FullName="System.Collections.Immutable.ImmutableArray&lt;T&gt;">
+<Type Name="ImmutableArray&lt;T&gt;" FullName="System.Collections.Immutable.ImmutableArray&lt;T&gt;">
   <TypeSignature Language="C#" Value="public struct ImmutableArray&lt;T&gt; : IEquatable&lt;System.Collections.Immutable.ImmutableArray&lt;T&gt;&gt;, System.Collections.Generic.ICollection&lt;T&gt;, System.Collections.Generic.IEnumerable&lt;T&gt;, System.Collections.Generic.IList&lt;T&gt;, System.Collections.Generic.IReadOnlyCollection&lt;T&gt;, System.Collections.Generic.IReadOnlyList&lt;T&gt;, System.Collections.IList, System.Collections.Immutable.IImmutableList&lt;T&gt;, System.Collections.IStructuralComparable, System.Collections.IStructuralEquatable" />
   <TypeSignature Language="ILAsm" Value=".class public sequential ansi sealed beforefieldinit ImmutableArray`1&lt;T&gt; extends System.ValueType implements class System.Collections.Generic.ICollection`1&lt;!T&gt;, class System.Collections.Generic.IEnumerable`1&lt;!T&gt;, class System.Collections.Generic.IList`1&lt;!T&gt;, class System.Collections.Generic.IReadOnlyCollection`1&lt;!T&gt;, class System.Collections.Generic.IReadOnlyList`1&lt;!T&gt;, class System.Collections.ICollection, class System.Collections.IEnumerable, class System.Collections.IList, class System.Collections.Immutable.IImmutableList`1&lt;!T&gt;, class System.Collections.IStructuralComparable, class System.Collections.IStructuralEquatable, class System.IEquatable`1&lt;valuetype System.Collections.Immutable.ImmutableArray`1&lt;!T&gt;&gt;" />
   <TypeSignature Language="DocId" Value="T:System.Collections.Immutable.ImmutableArray`1" />
@@ -325,9 +325,9 @@
       </TypeParameters>
       <Parameters />
       <Docs>
-        <typeparam name="TOther">To be added.</typeparam>
+        <typeparam name="TOther">The type of array element to return.</typeparam>
         <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct by casting the underlying array to an array of type <typeparamref name="TOther" />.</summary>
-        <returns>To be added.</returns>
+        <returns>An immutable array instance with elements cast to the new type.</returns>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidCastException">Thrown if the cast is illegal.</exception>
       </Docs>
@@ -366,10 +366,10 @@
         <Parameter Name="items" Type="System.Collections.Immutable.ImmutableArray&lt;TDerived&gt;" />
       </Parameters>
       <Docs>
-        <typeparam name="TDerived">To be added.</typeparam>
+        <typeparam name="TDerived">The type of array element to return.</typeparam>
         <param name="items">The array to initialize the array with. No copy is made.</param>
-        <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct based on the contents             of an existing instance, allowing a covariant static cast to efficiently reuse the existing array.</summary>
-        <returns>To be added.</returns>
+        <summary>Initializes a new instance of the <see cref="T:System.Collections.Immutable.ImmutableArray`1" /> struct based on the contents of an existing instance, allowing a covariant static cast to efficiently reuse the existing array.</summary>
+        <returns>An immutable array instance with elements cast to the new type.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -2152,8 +2152,8 @@
         <ReturnType>System.Int32</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the number of array in the collection.</summary>
-        <value>To be added.</value>
+        <summary>Gets the number of items in the collection.</summary>
+        <value>Number of items in the collection.</value>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidOperationException">Thrown if the <see cref="P:System.Collections.Immutable.ImmutableArray`1.IsDefault" /> property returns true.</exception>
       </Docs>
@@ -2421,8 +2421,8 @@
         <ReturnType>System.Int32</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the number of array in the collection.</summary>
-        <value>To be added.</value>
+        <summary>Gets the number of items in the collection.</summary>
+        <value>The number of items in the collection.</value>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidOperationException">Thrown if the <see cref="P:System.Collections.Immutable.ImmutableArray`1.IsDefault" /> property returns true.</exception>
       </Docs>
@@ -2540,7 +2540,7 @@
       </ReturnValue>
       <Docs>
         <summary>Gets the size of the array.</summary>
-        <value>To be added.</value>
+        <value>The number of items in the collection.</value>
         <remarks>To be added.</remarks>
         <exception cref="T:System.InvalidOperationException">Thrown if the <see cref="P:System.Collections.Immutable.ImmutableArray`1.IsDefault" /> property returns true.</exception>
       </Docs>
@@ -2576,9 +2576,17 @@
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>See the <see cref="T:System.Collections.ICollection" /> interface.</summary>
-        <value>To be added.</value>
-        <remarks>To be added.</remarks>
+        <summary>See the <see cref="T:System.Collections.ICollection" /> interface. Always returns <see langword="true" /> since since immutable collections are thread-safe.</summary>
+        <value>Boolean value determining whether the collection is thread-safe.</value>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This member is an explicit interface member implementation. It can be used only when the <xref:System.Collections.Immutable.ImmutableArray`1> instance is cast to an <xref:System.Collections.ICollection> interface.
+
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="System.Collections.ICollection.SyncRoot">
@@ -2613,7 +2621,7 @@
       </ReturnValue>
       <Docs>
         <summary>Gets the sync root.</summary>
-        <value>To be added.</value>
+        <value>An object for synchronizing access to the collection.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>