[3.13] gh-109070: Document that get_context in multiprocessing have side effect (GH-136341) (GH-136344)

gh-109070: Document that get_context in multiprocessing have side effect (GH-136341)

Document that get_context in multiprocessing have side effect
(cherry picked from commit 77a8bd2)

Co-authored-by: AN Long <[email protected]>

Author: miss-islington

Doc/library/multiprocessing.rst

@@ -1079,7 +1079,9 @@ Miscellaneous
    Return a context object which has the same attributes as the
    :mod:`multiprocessing` module.
 
-   If *method* is ``None`` then the default context is returned.
+   If *method* is ``None`` then the default context is returned. Note that if
+   the global start method has not been set, this will set it to the
+   default method.
    Otherwise *method* should be ``'fork'``, ``'spawn'``,
    ``'forkserver'``.  :exc:`ValueError` is raised if the specified
    start method is not available.  See :ref:`multiprocessing-start-methods`.
@@ -1090,10 +1092,10 @@ Miscellaneous
 
    Return the name of start method used for starting processes.
 
-   If the start method has not been fixed and *allow_none* is false,
-   then the start method is fixed to the default and the name is
-   returned.  If the start method has not been fixed and *allow_none*
-   is true then ``None`` is returned.
+   If the global start method has not been set and *allow_none* is
+   ``False``, then the start method is set to the default and the name
+   is returned. If the start method has not been set and *allow_none* is
+   ``True`` then ``None`` is returned.
 
    The return value can be ``'fork'``, ``'spawn'``, ``'forkserver'``
    or ``None``.  See :ref:`multiprocessing-start-methods`.