[docs] [issue27992] Clarify %(prog)s in argparse help formatter returns basename of sys.argv[0] by default

[py.user]

Change by py.user <bugzilla-mail-box at yandex.ru>:


----------
pull_requests: +12642
stage:  -> patch review

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue27992>
_______________________________________