message: Clarify non-obvious call to unbecome
diff: @@ -250,6 +250,8 @@ expected<group> middleman::remote_group(const std::string& group_identifier,
  middleman_actor mm) -> behavior {
  return {
  [=](get_atom) {
  + /// We won't receive a second message, so we drop our behavior here to
  + /// terminate the actor after both requests finish.
  self->unbecome();
  auto rp = self->make_response_promise();
  self->request(mm, infinite, connect_atom::value, host, port).then(
  