[issue13386] Document documentation conventions for optional args

[Ezio Melotti]

Ezio Melotti <ezio.melotti at gmail.com> added the comment:

The problem is when the default placeholder is some unique object() or some _internal value (we had something similar with a socket timeout once).
Also for something like str.strip(), would you document chars=None or chars=" \n\r\t\v\f"?

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