[3.13] gh-80334: fix multiprocessing.freeze_support for other spawn platforms (GH-134462) (#134620)

gh-80334: fix multiprocessing.freeze_support for other spawn platforms (GH-134462)

Doc/library/multiprocessing.rst: freeze_support: Change to specify spawn method instead of platform
Have multiprocessing.freeze_support() enable on spawn, not just win32.

---------
(cherry picked from commit 80284b5c5eebd0e603c38322f94a97a2853ceeba)

Co-authored-by: Eddy Mulyono <eddymul@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>