[3.13] gh-84559: improve What's New entry for multiprocessing startmethod changes (GH-128173) (#137156)

[3.13] gh-84559: improve What's New entry for `multiprocessing` start method changes (GH-128173)
(cherry picked from commit b8c313a)

Co-authored-by: Bénédikt Tran <[email protected]>
Co-authored-by: Gregory P. Smith <[email protected]>
Co-authored-by: Stan Ulbrych <[email protected]>

Author: 4 people

Doc/library/multiprocessing.rst

@@ -111,6 +111,8 @@ Contexts and start methods
 Depending on the platform, :mod:`multiprocessing` supports three ways
 to start a process.  These *start methods* are
 
+  .. _multiprocessing-start-method-spawn:
+
   *spawn*
     The parent process starts a fresh Python interpreter process.  The
     child process will only inherit those resources necessary to run
@@ -121,6 +123,8 @@ to start a process.  These *start methods* are
 
     Available on POSIX and Windows platforms.  The default on Windows and macOS.
 
+  .. _multiprocessing-start-method-fork:
+
   *fork*
     The parent process uses :func:`os.fork` to fork the Python
     interpreter.  The child process, when it begins, is effectively
@@ -141,6 +145,8 @@ to start a process.  These *start methods* are
        raise a :exc:`DeprecationWarning`. Use a different start method.
        See the :func:`os.fork` documentation for further explanation.
 
+  .. _multiprocessing-start-method-forkserver:
+
   *forkserver*
     When the program starts and selects the *forkserver* start method,
     a server process is spawned.  From then on, whenever a new process
@@ -3025,6 +3031,9 @@ Beware of replacing :data:`sys.stdin` with a "file like object"
 
     For more information, see :issue:`5155`, :issue:`5313` and :issue:`5331`
 
+.. _multiprocessing-programming-spawn:
+.. _multiprocessing-programming-forkserver:
+
 The *spawn* and *forkserver* start methods
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^