[docs] [issue18384] Add devhelp build instructions to the documentation makefile

[Ezio Melotti]

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


----------
nosy: +georg.brandl
stage:  -> patch review
versions: +Python 3.4 -Python 3.5

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue18384>
_______________________________________