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

Changes by Davin Potts <python@discontinuity.net>: Added file: http://bugs.python.org/file41331/issue_5501_explicit_freeze_support_nonWin_v... _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue5501> _______________________________________