Doc: Explain that multiprocessing only supports unpicklable functions through the fork method

In multiprocessing and concurrent.futures's documentations, as well as
the release notes for Python 3.14.

This moves the paragraph in 3.14's release notes from the deprecations
section to 'Porting to Python 3.14' because it is a breaking change,
which follows a deprecation in Python 3.12.

This explicitly mentions `PicklingError` in this section, so it is easy
to find for somehow who gets the error and searches its name through
the page.

Author: progval

Doc/library/concurrent.futures.rst

@@ -412,7 +412,8 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
    .. versionchanged:: 3.14
       The default process start method (see
       :ref:`multiprocessing-start-methods`) changed away from *fork*. If you
-      require the *fork* start method for :class:`ProcessPoolExecutor` you must
+      require the *fork* start method for :class:`ProcessPoolExecutor` or need to run
+      :ref:`unpicklable functions <pickle-picklable>`, you must
       explicitly pass ``mp_context=multiprocessing.get_context("fork")``.
 
 .. _processpoolexecutor-example:

Doc/library/multiprocessing.rst

@@ -115,6 +115,8 @@ to start a process.  These *start methods* are
     will not be inherited.  Starting a process using this method is
     rather slow compared to using *fork* or *forkserver*.
 
+    Functions run with this method must be :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX and Windows platforms.  The default on Windows and macOS.
 
   *fork*
@@ -124,6 +126,9 @@ to start a process.  These *start methods* are
     inherited by the child process.  Note that safely forking a
     multithreaded process is problematic.
 
+    Functions run with this method do *not* need to be
+    :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX systems.
 
     .. versionchanged:: 3.14
@@ -146,6 +151,8 @@ to start a process.  These *start methods* are
     side-effect so it is generally safe for it to use :func:`os.fork`.
     No unnecessary resources are inherited.
 
+    Functions run with this method must be :ref:`picklable <pickle-picklable>`.
+
     Available on POSIX platforms which support passing file descriptors over
     Unix pipes such as Linux.  The default on those.
 

Doc/whatsnew/3.14.rst

@@ -451,14 +451,6 @@ Deprecated
   as a single positional argument.
   (Contributed by Serhiy Storchaka in :gh:`109218`.)
 
-* :mod:`multiprocessing` and :mod:`concurrent.futures`:
-  The default start method (see :ref:`multiprocessing-start-methods`) changed
-  away from *fork* to *forkserver* on platforms where it was not already
-  *spawn* (Windows & macOS). If you require the threading incompatible *fork*
-  start method you must explicitly specify it when using :mod:`multiprocessing`
-  or :mod:`concurrent.futures` APIs.
-  (Contributed by Gregory P. Smith in :gh:`84559`.)
-
 * :mod:`os`:
   :term:`Soft deprecate <soft deprecated>` :func:`os.popen` and
   :func:`os.spawn* <os.spawnl>` functions. They should no longer be used to
@@ -625,6 +617,8 @@ Others
   :meth:`~object.__index__`. (Contributed by Mark Dickinson in :gh:`119743`.)
 
 
+.. _whatsnew314-porting-to-python314:
+
 Porting to Python 3.14
 ======================
 
@@ -643,6 +637,17 @@ Changes in the Python API
   This temporary change affects other threads.
   (Contributed by Serhiy Storchaka in :gh:`69998`.)
 
+* :mod:`multiprocessing` and :mod:`concurrent.futures`:
+  The default start method (see :ref:`multiprocessing-start-methods`) changed
+  away from *fork* to *forkserver* on platforms where it was not already
+  *spawn* (Windows & macOS). If you require the threading incompatible *fork*,
+  which can run :ref:`unpicklable functions <pickle-picklable>`,
+  start method you must explicitly specify it when using :mod:`multiprocessing`
+  or :mod:`concurrent.futures` APIs.
+  Code running pickable functions on these platforms without explicitly opting
+  into the *fork* method will now raise :exc:`pickle.PicklingError`.
+  (Contributed by Gregory P. Smith in :gh:`84559`.)
+
 
 Build changes
 =============