[3.12] gh-100228: Document the os.fork threads DeprecationWarning. (GH-109767) (#109773)

* gh-100228: Document the os.fork threads DeprecationWarning. (GH-109767)

Document the `os.fork` posix threads detected `DeprecationWarning` in 3.12 What's New, os, multiprocessing, and concurrent.futures docs.

Many reviews and doc cleanup edits by Adam & Hugo. 🥳

(cherry picked from commit 5e7ea95d9d5c3b80a67ffbeebd76ce4fc327dd8e)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

* link to the discussion thread from whatsnew

Include the link to the discussion in the what's new text per @malemberg's comment on. https://github.com/python/cpython/pull/109767

(i'll follow up with a PR to main to include this edit there as well)

---------

Co-authored-by: Gregory P. Smith <greg@krypto.org>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>