[docs] bug report: multiprocessing module

[Greenpickle]

In particular, the section on Windows (16.6.3.2, the version at
http://docs.python.org/library/multiprocessing.html#windows, if there happen to
be others).  It mentions that the 'main module' must be able to be 'safely
imported', but doesn't make it clear that it needs the .py extension, so that
it can be imported at all.

Just thought it might help others where I spent a few minutes trying to work
out what was going wrong.