Document System.Xml.Linq.XComment.WriteToAsync (#3399)

Author: carlossanlop

xml/System.Xml.Linq/XComment.xml

@@ -381,7 +381,7 @@ This is a comment
         <param name="writer">The writer to write this <see cref="T:System.Xml.Linq.XComment" /> to.</param>
         <param name="cancellationToken">A token that can be used to request cancellation of the asynchronous operation.</param>
         <summary>Writes this <see cref="T:System.Xml.Linq.XComment" /> to the specified <see cref="T:System.Xml.XmlWriter" />.</summary>
-        <returns>To be added.</returns>
+        <returns>A task that represents the asynchronous write operation.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>