gh-91340: Document multiprocessing.set_start_method force parameter (GH-32339)

#91340


https://bugs.python.org/issue47184

Automerge-Triggered-By: GH:kumaraditya303

Author: dignissimus

Doc/library/multiprocessing.rst

@@ -1089,10 +1089,14 @@ Miscellaneous
    .. versionchanged:: 3.11
       Accepts a :term:`path-like object`.
 
-.. function:: set_start_method(method)
+.. function:: set_start_method(method, force=False)
 
    Set the method which should be used to start child processes.
-   *method* can be ``'fork'``, ``'spawn'`` or ``'forkserver'``.
+   The *method* argument can be ``'fork'``, ``'spawn'`` or ``'forkserver'``.
+   Raises :exc:`RuntimeError` if the start method has already been set and *force*
+   is not ``True``.  If *method* is ``None`` and *force* is ``True`` then the start
+   method is set to ``None``.  If *method* is ``None`` and *force* is ``False``
+   then the context is set to the default context.
 
    Note that this should be called at most once, and it should be
    protected inside the ``if __name__ == '__main__'`` clause of the