Arvados

While looking at the documentation for potential spots that still need tweaking, I think I've found some traces of a currently not supported feature: the async flag on the groups API:

• API documentation mentions it along with the async_permissions_update_interval config knob.
• Some test & config code is still existing about API.AsyncPermissionsUpdateInterval and its relationship with the previous point.
• Group's controller and arvados model still have some code related the async parameter, but it seems to me that we don't really do anything with it anymore.

(copied from #18277#note-16)