Add an explanatory comment to DuplexPipe (dotnet#56863)

Author: amcasey

src/Shared/ServerInfrastructure/DuplexPipe.cs

@@ -3,6 +3,19 @@
 
 namespace System.IO.Pipelines;
 
+/// <remarks>
+/// A <see cref="Pipe"/> is a reader-writer pair, where content written to the writer can be read from the reader.
+///
+/// An <see cref="IDuplexPipe"/> is *not* a pipe.  It is also a reader-writer pair, but the reader and writer are not
+/// connected.  Rather, it can be regarded as *one end* of a two-way (i.e. duplex) communication channel, where content
+/// written to the writer is sent to the counterparty and content received from the counterparty is readable from the
+/// reader.
+///
+/// A <see cref="DuplexPipePair"/> is a pair of <see cref="IDuplexPipe"/> instances, each of which represents one end of
+/// a two-way communication channel.  (In a sense, this makes it a "duplex pipe".)  It can also be viewed as a pair of
+/// <see cref="Pipe"/>s, as these underlie the <see cref="IDuplexPipe"/> instances.  In either view, it is composed of
+/// two <see cref="PipeReader"/>s and two <see cref="PipeWriter"/>s - it is only how they are grouped that differs.
+/// </remarks>
 internal sealed class DuplexPipe : IDuplexPipe
 {
     public DuplexPipe(PipeReader reader, PipeWriter writer)