message: Clarify poll_message docs
  Closes
diff: @@ -302,6 +302,9 @@ where
  ///
  /// The server can send notices as well as notifications asynchronously to the client. Applications that wish to
  /// examine those messages should use this method to drive the connection rather than its `Future` implementation.
  + ///
  + /// Return values of `None` or `Some(Err(_))` are "terminal"; callers should not invoke this method again after
  + /// receiving one of those values.
  pub fn poll_message(
  &mut self,
  cx: &mut Context<'_>,
  