Improve docs for setup_all + on_exit (#11823)
Improve docs for setup_all + on_exit (#11823)

I made a pass over the docs for "ExUnit.Callbacks.setup_all/1" and
"ExUnit.Callbacks.on_exit/2". I mostly clarified the behavior of
"on_exit/2" when called from "setup_all/1".
michalmuskala/elixir,elixir-lang/elixir