title: How Elixir Supervisor stops children on stop?
body: <p>I have a supervisor with <code>:simple_one_to_one</code> restart strategy, what happens with children when I call <code>Supervisor.stop(sup)</code>?</p>
  
  <p>From the tests I see that they're dying no matter what they were doing. Is there standard way to shut them down gracefully, so that they could finish the job (if any)? Something like calling <code>GenServer.stop</code> on them...</p>