Mention that filters_from_request can return an async function

[cycsmail]

Closes:

• filters_from_request plugin hook docs should mention that returning an async function is allowed #1638

The filters_from_request hook can return an async def function (Datasette calls it and awaits the result via await_me_maybe), so you can run SQL or other async work while building your filters. The docs didn't mention this, so I added a short note to the filters_from_request section pointing it out and linking to the default implementations in datasette/filters.py.

Also added a small test_docs.py check that the documented section mentions async def; it fails without the doc change and passes with it.