Document context expirations wart

Closes #242

conn.go

@@ -37,6 +37,9 @@ const (
 //
 // On any error from any method, the connection is closed
 // with an appropriate reason.
+//
+// This applies to context expirations as well unfortunately.
+// See https://github.com/nhooyr/websocket/issues/242#issuecomment-633182220
 type Conn struct {
 	subprotocol    string
 	rwc            io.ReadWriteCloser