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

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


----------
nosy: +ezio.melotti
stage:  -> patch review
type:  -> enhancement

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