`loop.call_soon_threadsafe` raises `RuntimeError` when the event loop has been closed, but that fact doesn't seem to be documented. It would be useful to document it so that that it's clear that that behaviour is part of the API, and can be depended on.

Doc link: I'm looking at https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.call_soon_threadsafe

My use-case is that I have a background thread that's making use of `loop.call_soon_threadsafe` to place callbacks onto the main thread's event loop. The main thread at some point closes that event loop (e.g., as part of controlled application shutdown, or in the tearDown of a unit test). The background thread isn't in a position to know whether the event loop has been closed or not, and obviously checking that using the `is_closed` attribute runs into race condition issues. So I'd like to catch the potential exception from the `loop.call_soon_threadsafe`, but to do that I need to know what exception type to catch.

It would probably also make sense to document the failure mode for `loop.call_soon`.

Thanks! I've opened a PR for this issue.

New changeset 3240bc62f4e0afa09964f3afc845697f0a0806b9 by meowmeowmeowcat in branch 'main':
bpo-44508: [Doc] Document failure mode for loop.call_soon_threadsafe (GH-27688)
https://github.com/python/cpython/commit/3240bc62f4e0afa09964f3afc845697f0a0806b9

New changeset bbb10761b64095425f7f8dcb3bc1f0d8bdee528a by Miss Islington (bot) in branch '3.10':
bpo-44508: [Doc] Document failure mode for loop.call_soon_threadsafe (GH-27688)
https://github.com/python/cpython/commit/bbb10761b64095425f7f8dcb3bc1f0d8bdee528a

New changeset e2320c6a34d44a0efa7ecc693c3a6215014b7b6e by Miss Islington (bot) in branch '3.9':
bpo-44508: [Doc] Document failure mode for loop.call_soon_threadsafe (GH-27688) (GH-27805)
https://github.com/python/cpython/commit/e2320c6a34d44a0efa7ecc693c3a6215014b7b6e

Thanks for reporting, Mark, and for the patch, meowmeowcat! ✨ 🍰 ✨