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

[hai shi]

hai shi <shihai1991 at 126.com> added the comment:

It exists in master, so it still need a patch.

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nosy: +rhettinger, shihai1991
versions: +Python 3.9 -Python 2.7, Python 3.2

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