message: Document the ready polling methods
diff: @@ -311,10 +311,13 @@ where
  req
  }
  + /// Poll whether this client connection is ready to send another request.
  pub fn poll_ready(&mut self) -> Poll<(), core_error::Error> {
  self.service.poll_ready().map_err(error_from_grpc)
  }
  + /// Get a `Future` of when this client connection is ready to send
  + /// another request.
  pub fn ready(self) -> impl Future<Item = Self, Error = core_error::Error> {
  let node_id = self.node_id;
  self.service
  