[docs] [issue23738] Clarify documentation of positional-only default values

[R. David Murray]

R. David Murray added the comment:

Personally I would rather stick to the [] syntax in the docs, with the default values mentioned in the text.

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nosy: +georg.brandl, r.david.murray

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue23738>
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