Make pagination of rooms in admin api stable (#11737)

Always add state.room_id after the configurable ORDER BY. Otherwise,
for any sort, certain pages can contain results from
other pages. (Especially when sorting by creator, since there may
be many rooms by the same creator)

* Document different order direction of numerical fields

"joined_members", "joined_local_members", "version" and "state_events"
are ordered in descending direction by default (dir=f). Added a note
in tests to explain the differences in ordering.

Signed-off-by: Daniël Sonck <daniel@sonck.nl>