[3.13] gh-85222: Document the global start method side effect in multiprocessing (GH-136426) (#142771)

gh-85222: Document the global start method side effect in multiprocessing (GH-136426)

* Document the ctx parameter in some types in multiprocessing.
* Reduce duplication while still linking to the central explanation from API points with the side effect.
(cherry picked from commit 0978b9a7d5eedab19d7d39bd703ed5b55fd7aa5f)

Co-authored-by: AN Long <aisk@users.noreply.github.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: R Chintan Meher <meherrihaan@gmail.com>