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.

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: R Chintan Meher <meherrihaan@gmail.com>