Clarify docs about requesting the `event_loop` fixture

[tuukkamustonen]

https://pytest-asyncio.readthedocs.io/en/latest/reference/fixtures/index.html#event-loop suggests that it's fine to grab event_loop fixture in a non-async function.

0.21+ migration guide in https://pytest-asyncio.readthedocs.io/en/latest/how-to-guides/migrate_from_0_21.html suggests to convert all sync functions requesting the event_loop fixture into async functions, and then acquire the loop via asyncio.get_running_loop().

Which one is it, and what is the rationale of not depending on the event_loop fixture directly?

[seifertm]

The use of _eventloop is deprecated. We should adjust the docs to reflect that. The rationale is summarized here: https://github.com/pytest-dev/pytest-asyncio/issues/706#issuecomment-2282780197