message: add clarifying comment on Tunnel::shutdown
diff: @@ -43,6 +43,9 @@ impl Tunnel {
  self.shutdown().await
  }
  + /// shutdown is relatively simple, it sends a second `kill` signal to the child process,
  + /// short-circuiting cloudflared's "graceful shutdown" period. this approach has been endorsed
  + /// by the team who maintains cloudflared as safe practice.
  pub async fn shutdown(mut self) -> Result<(), failure::Error> {
  let pid = self.child.id();
  if let Err(e) = self.child.kill() {
  