`filters_from_request` plugin hook docs should mention that returning an async function is allowed

[simonw]

https://docs.datasette.io/en/stable/plugin_hooks.html#filters-from-request-request-database-table-datasette doesn't mention that you can return an async function - but you can, and in fact Datasette itself uses that here: https://github.com/simonw/datasette/blob/aa7f0037a46eb76ae6fe9bf2a1f616c58738ecdf/datasette/filters.py#L43-L47