gh-109070: multiprocessing.get_context will not set the start method globally

[aisk]

• Issue: Document that "multiprocessing.get_context()" is also setting the context globally #109070

[auvipy]

looks good

[gpshead]

I wish I could say "or perhaps even just return self._default_context?" given the documentation for .get_context() today. BUT code relies on .get_context() to establish the actual set context and thus prevent it from being changed without force=True so that different contexts are not used between API surfaces at the default module level (Lib/multiprocessing/__init__.py sets module level attributes to be the multiprocessing.context._default_context instance attributes)

So I think we really are best off just documenting the existing behavior. As is already done for get_start_method() in the docs right below get_context() - https://docs.python.org/3/library/multiprocessing.html
Examples: All of the things defined in BaseContext that when used have a ctx=self.get_context() call in them. Once something is used with the default context we don't want to allow a set_start_method call to change it.