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

[Martin Panter]

Changes by Martin Panter <vadmium+py at gmail.com>:


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nosy: +berker.peksag
versions: +Python 3.6
Added file: http://bugs.python.org/file40417/pos-defaults.v3.patch

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