message: add some comments to the network component
diff: @@ -401,12 +401,18 @@ async fn server_task<REv: ReactorEventT<P>, P: PayloadT>(
  let our_id_cloned = our_id.clone();
  let main_task = async move {
  loop {
  + // Note that `select!` will cancel all futures on branches not eventually selected by
  + // dropping them. Each future inside this macro must be cancellation-safe.
  select! {
  + // `swarm.next_event()` is cancellation-safe - see
  + // https://github.com/libp2p/rust-libp2p/issues/1876
  swarm_event = swarm.next_event() => {
  trace!("{}: {:?}", our_id_cloned, swarm_event);
  handle_swarm_event(event_queue, swarm_event).await;
  }
  + // `UnboundedReceiver::recv()` is also cancellation safe - see
  + // https://tokio.rs/tokio/tutorial/select#cancellation
  maybe_outgoing_message = one_way_outgoing_message_receiver.recv() => {
  match maybe_outgoing_message {
  Some(outgoing_message) => {
  