Document caution regarding AdvanceTo's side effect (#3659)

* Document caution regarding AdvanceTo's side effect

After calling `PipeReader.AdvanceTo`, the `ReadResult.Buffer` property's backing data is recycled, leading to indeterministic changes to not only the content but its `Length` property. This isn't particular predictable as a consumer and the docs didn't make it clear either.

* Use xref's

Co-Authored-By: Bill Wagner <[email protected]>

Author: AArnott

xml/System.IO.Pipelines/PipeReader.xml

@@ -53,7 +53,7 @@
 
 ## Remarks
 
-The memory for the consumed data will be released and no longer available. The examined data communicates to the pipeline when it should signal more data is available.
+The memory for the consumed data will be released and no longer available. The <xref:System.IO.Pipelines.ReadResult.Buffer%2A> previously returned from <xref:System.IO.Pipelines.PipeReader.ReadAsync(System.Threading.CancellationToken)> must not be accessed after this call. The examined data communicates to the pipeline when it should signal more data is available.
 
           ]]></format>
         </remarks>