[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>

1 files changed, 6 insertions, 0 deletions

diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 38d24a8..2f0f1f8 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -131,6 +131,12 @@ to start a process.  These *start methods* are
        Code that requires *fork* should explicitly specify that via
        :func:`get_context` or :func:`set_start_method`.
 
+    .. versionchanged:: 3.12
+       If Python is able to detect that your process has multiple threads, the
+       :func:`os.fork` function that this start method calls internally will
+       raise a :exc:`DeprecationWarning`. Use a different start method.
+       See the :func:`os.fork` documentation for further explanation.
+
   *forkserver*
     When the program starts and selects the *forkserver* start method,
     a server process is spawned.  From then on, whenever a new process