Add read state transitions info

src/libraries/System.Net.Quic/src/System/Net/Quic/Implementations/MsQuic/MsQuicStream.cs

@@ -1411,6 +1411,16 @@ private static uint HandleEventConnectionClose(State state)
         private static Exception GetConnectionAbortedException(State state) =>
             ThrowHelper.GetConnectionAbortedException(state.ConnectionState.AbortErrorCode);
 
+        // Read state transitions:
+        //
+        // None  --(data arrives in event RECV)->  IndividualReadComplete  --(user calls ReadAsync() & completes syncronously)->  None
+        // None  --(user calls ReadAsync() & waits)->  PendingRead  --(data arrives in event RECV & completes user's ReadAsync())->  None
+        // Any non-final state  --(event PEER_SEND_SHUTDOWN or SHUTDOWN_COMPLETED with ConnectionClosed=false)->  ReadsCompleted
+        // Any non-final state  --(event PEER_SEND_ABORT)->  Aborted
+        // Any non-final state  --(user calls AbortRead())->  Aborted
+        // Any state  --(CancellationToken's cancellation for ReadAsync())->  Aborted (TODO: should it be only for non-final as others?) 
+        // Any non-final state  --(event SHUTDOWN_COMPLETED with ConnectionClosed=true)->  ConnectionClosed
+        // Closed - no transitions, set for Unidirectional write-only streams
         private enum ReadState
         {
             /// <summary>
@@ -1428,6 +1438,8 @@ private enum ReadState
             /// </summary>
             PendingRead,
 
+            // following states are final:
+
             /// <summary>
             /// The peer has gracefully shutdown their sends / our receives; the stream's reads are complete.
             /// </summary>