Fix misleading wording in PipeReader and PipeWriter's AsStream documentation. (#4156)

* Fix wording in PipeReader's AsStream docs

The old wording gets it exactly wrong - the PipeReader doesn't wrap the Stream, the Stream wraps the PipeReader.

Co-Authored-By: Stephen Halter <[email protected]>

Author: Benjamin Hodgson

xml/System.IO.Pipelines/PipeReader.xml

@@ -166,8 +166,8 @@ The examined data communicates to the pipeline when it should signal more data i
       </Parameters>
       <Docs>
         <param name="leaveOpen">An optional flag that indicates whether disposing the returned <see cref="T:System.IO.Stream" /> leaves <see cref="T:System.IO.Pipelines.PipeReader" /> open (<see langword="true" />) or completes <see cref="T:System.IO.Pipelines.PipeReader" /> (<see langword="false" />).</param>
-        <summary>Returns the underlying <see cref="T:System.IO.Stream" /> wrapped by the <see cref="T:System.IO.Pipelines.PipeReader" />.</summary>
-        <returns>The underlying stream.</returns>
+        <summary>Returns a <see cref="T:System.IO.Stream" /> representation of the <see cref="T:System.IO.Pipelines.PipeReader" />.</summary>
+        <returns>A stream that represents the <see cref="T:System.IO.Pipelines.PipeReader" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>

xml/System.IO.Pipelines/PipeWriter.xml

@@ -100,8 +100,8 @@
       </Parameters>
       <Docs>
         <param name="leaveOpen">An optional flag that indicates whether disposing the returned <see cref="T:System.IO.Stream" /> leaves <see cref="T:System.IO.Pipelines.PipeReader" /> open (<see langword="true" />) or completes <see cref="T:System.IO.Pipelines.PipeReader" /> (<see langword="false" />).</param>
-        <summary> Returns the underlying <see cref="T:System.IO.Stream" /> wrapped by the <see cref="T:System.IO.Pipelines.PipeWriter" />.</summary>
-        <returns>The underlying stream.</returns>
+        <summary>Returns a <see cref="T:System.IO.Stream" /> representation of the <see cref="T:System.IO.Pipelines.PipeWriter" />.</summary>
+        <returns>A stream that represents the <see cref="T:System.IO.Pipelines.PipeWriter" />.</returns>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>