gh-109070: Document that multiprocessing.get_context() sets the global context

Doc/library/multiprocessing.rst

@@ -1058,24 +1058,40 @@ 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.
-   Otherwise *method* should be ``'fork'``, ``'spawn'``,
-   ``'forkserver'``.  :exc:`ValueError` is raised if the specified
-   start method is not available.  See :ref:`multiprocessing-start-methods`.
+   If *method* is ``None``, the default context is returned. Be aware that
+   calling this function with *method=None* sets the default context globally,
+   potentially affecting future usage of :func:`set_start_method`. To change
+   the method later, it is recommended to explicitly specify a method other
+   than ``None`` when calling this function. Otherwise, it may lead to
+   unexpected behavior.
+
+   *method* should be one of the following: ``'fork'``, ``'spawn'``, or
+   ``'forkserver'``. A :exc:`ValueError` is raised if the specified start
+   method is unavailable. See :ref:`multiprocessing-start-methods`.
 
    .. versionadded:: 3.4
 
 .. function:: get_start_method(allow_none=False)
 
    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 start method has not been explicitly set
+   (e.g., using :func:`set_start_method`) and *allow_none* is false, then the
+   start method is fixed to the default, and the name of the default method
+   is returned. If the start method has not been explicitly set and
+   *allow_none* is true, then ``None`` is returned, indicating that
+   the method has not been explicitly set.
 
    The return value can be ``'fork'``, ``'spawn'``, ``'forkserver'``
-   or ``None``.  See :ref:`multiprocessing-start-methods`.
+   or ``None``.
+
+   Exercise caution when changing the start method using 
+   ``multiprocessing.set_start_method`` after using
+   ``multiprocessing.get_start_method`` with *allow_none* set to
+   ``False``, as it may raise a ``RuntimeError``.
+
+   See :ref:`multiprocessing-start-methods` for more information
+   on multiprocessing start methods and their implications.
 
 .. versionchanged:: 3.8