[docs] [issue24632] Improve documentation about __main__.py

[Ezio Melotti]

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


----------
nosy: +ncoghlan
versions: +Python 3.6 -Python 3.4

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue24632>
_______________________________________