fix documentation for CancelWrite after Close on the send stream (#4419)

interface.go

@@ -124,7 +124,9 @@ type SendStream interface {
 	// CancelWrite aborts sending on this stream.
 	// Data already written, but not yet delivered to the peer is not guaranteed to be delivered reliably.
 	// Write will unblock immediately, and future calls to Write will fail.
-	// When called multiple times or after closing the stream it is a no-op.
+	// When called multiple times it is a no-op.
+	// When called after Close, it aborts delivery. Note that there is no guarantee if
+	// the peer will receive the FIN or the reset first.
 	CancelWrite(StreamErrorCode)
 	// The Context is canceled as soon as the write-side of the stream is closed.
 	// This happens when Close() or CancelWrite() is called, or when the peer