[docs] [issue9182] document “--” as a way to distinguish option w/ narg='+' from positional argument in argparse

hai shi <shihai1991@126.com> added the comment: It exists in master, so it still need a patch. ---------- nosy: +rhettinger, shihai1991 versions: +Python 3.9 -Python 2.7, Python 3.2 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue9182> _______________________________________