tweak asyncio.Runner loop_factory docs to emphasize that the default case *does* call `set_event_loop()`

[graingert]

Whatever gives users the best guidance. I searched for code that uses Runner(loop_factory=...) and there is one case apart from test_runners.py: in test_asyncio/test_subprocess.py it us used as loop_factory=asyncio.new_event_loop specifically to prevent the watcher to be set (since the test sets its own watcher).

I would emphasize that the default case does call set_event_loop() and implies attaching the watcher (if there is one!) to the loop; it's fine to suggest that loop_factory should be used only if this is undesired.

Originally posted by @gvanrossum in https://github.com/python/cpython/pull/95979#discussion_r946971337