[3.13] gh-109070: Document that get_context in multiprocessing have side effect (GH-136341) (GH-136344)

gh-109070: Document that get_context in multiprocessing have side effect (GH-136341)

Document that get_context in multiprocessing have side effect
(cherry picked from commit 77a8bd29da99e7d4fa8e7f07c4063977c2bb14d3)

Co-authored-by: AN Long <aisk@users.noreply.github.com>