Added useful details to PipeStream.ReadMode (#2004)

* Added useful details to `PipeStream.ReadMode`

* Changed remark to emphasize checking IsConnected before accessing ReadMode

Author: peteraritchie

xml/System.IO.Pipes/PipeStream.xml

@@ -1521,6 +1521,8 @@
   
 ## Remarks  
  Anonymous pipes do not support <xref:System.IO.Pipes.PipeTransmissionMode.Message> mode message transmission.  
+
+ To avoid an <xref:System.InvalidOperationException> accessing `ReadMode`, <xref:System.IO.Pipes.PipeStream.IsConnected> can be used to verify the pipe is connected.
   
  ]]></format>
         </remarks>
@@ -2039,4 +2041,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>