Add missing Zip API docs (#2816)

* Add missing Zip api docs

* Improvements based on the PR feedbacks

Author: maryamariyan

xml/System.Linq/Enumerable.xml

@@ -12608,12 +12608,12 @@ Only unique elements are returned.
         <Parameter Name="second" Type="System.Collections.Generic.IEnumerable&lt;TSecond&gt;" Index="1" FrameworkAlternate="netcore-3.0" />
       </Parameters>
       <Docs>
-        <typeparam name="TFirst">To be added.</typeparam>
-        <typeparam name="TSecond">To be added.</typeparam>
-        <param name="first">To be added.</param>
-        <param name="second">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <typeparam name="TFirst">The type of the elements of the first input sequence.</typeparam>
+        <typeparam name="TSecond">The type of the elements of the second input sequence.</typeparam>
+        <param name="first">The first sequence to merge.</param>
+        <param name="second">The second sequence to merge.</param>
+        <summary>Produces a sequence of tuples with elements from the two specified sequences.</summary>
+        <returns>A sequence of tuples with elements taken from the first and second sequences, in that order.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.Linq/Queryable.xml

@@ -8297,12 +8297,12 @@
         <Parameter Name="source2" Type="System.Collections.Generic.IEnumerable&lt;TSecond&gt;" Index="1" FrameworkAlternate="netcore-3.0" />
       </Parameters>
       <Docs>
-        <typeparam name="TFirst">To be added.</typeparam>
-        <typeparam name="TSecond">To be added.</typeparam>
-        <param name="source1">To be added.</param>
-        <param name="source2">To be added.</param>
-        <summary>To be added.</summary>
-        <returns>To be added.</returns>
+        <typeparam name="TFirst">The type of the elements of the first input sequence.</typeparam>
+        <typeparam name="TSecond">The type of the elements of the second input sequence.</typeparam>
+        <param name="source1">The first sequence to merge.</param>
+        <param name="source2">The second sequence to merge.</param>
+        <summary>Produces a sequence of tuples with elements from the two specified sequences.</summary>
+        <returns>A sequence of tuples with elements taken from the first and second sequences, in that order.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>