Document StringBuilder.Append (#2820)

Author: carlossanlop

xml/System.Text/StringBuilder.xml

@@ -1336,9 +1336,9 @@
         <Parameter Name="value" Type="System.ReadOnlyMemory&lt;System.Char&gt;" Index="0" FrameworkAlternate="netcore-3.0" />
       </Parameters>
       <Docs>
-        <param name="value">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="value">The read-only character memory region to append.</param>
+        <summary>Appends the string representation of a specified read-only character memory region to this instance.</summary>
+        <returns>A reference to this instance after the append operation is completed.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -1368,9 +1368,9 @@
         <Parameter Name="value" Type="System.ReadOnlySpan&lt;System.Char&gt;" Index="0" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="value">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="value">The read-only character span to append.</param>
+        <summary>Appends the string representation of a specified read-only character span to this instance.</summary>
+        <returns>A reference to this instance after the append operation is completed.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -1593,9 +1593,9 @@
         <Parameter Name="value" Type="System.Text.StringBuilder" Index="0" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="value">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="value">The string builder to append.</param>
+        <summary>Appends the string representation of a specified string builder to this instance.</summary>
+        <returns>A reference to this instance after the append operation is completed.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
@@ -2130,11 +2130,11 @@
         <Parameter Name="count" Type="System.Int32" Index="2" FrameworkAlternate="netcore-2.1;netcore-2.2;netcore-3.0;netstandard-2.1" />
       </Parameters>
       <Docs>
-        <param name="value">To be added.</param>
-        <param name="startIndex">To be added.</param>
-        <param name="count">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <param name="value">The string builder that contains the substring to append.</param>
+        <param name="startIndex">The starting position of the substring within <paramref name="value" />.</param>
+        <param name="count">The number of characters in <paramref name="value" /> to append.</param>
+        <summary>Appends a copy of a substring within a specified string builder to this instance.</summary>
+        <returns>A reference to this instance after the append operation has completed.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>