[docs] [issue22867] document behavior of calling atexit.register() while atexit._run_exitfuncs is running

[Ethan Furman]

Changes by Ethan Furman <ethan at stoneleaf.us>:


----------
nosy: +ethan.furman

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22867>
_______________________________________