[docs] [issue5501] Update multiprocessing docs re: freeze_support

[Éric Araujo]

Éric Araujo <merwok at netwok.org> added the comment:

I’m assuming this is a feature new in 2.7 and 3.2, please add 3.1 if this is wrong.

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nosy: +docs at python, eric.araujo

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