Add better method to repopulate room stats than poking the DB (or remove docs).

[erikjohnston]

As per #8238 we have docs that say to repopulate the room stats table you can insert populate_stats_process_rooms into the background job table. This is a bit icky as a) its not obvious that background jobs are a "public API" and b) if we add further background jobs that depend on populate_stats_process_rooms those will not get re-run (which might lead to bugs).

This feels like something that should be an admin API, or some other form of explicit API, to avoid the above issues.

(Note: we do have tests that manually inserting populate_stats_process_rooms does the Right Thing, but since its not obvious that its a "public API" the tests may just be modified, which is what allowed the regression in #8238)