Improve commentary about hack in is_publishable_class().
The FirstNormalObjectId test here is a kluge that needs to go away,
but the only substitute we can think of is to add a column to pg_class,
which will take more work than can be handled right now. Add some
commentary in the meanwhile.
Discussion: https://postgr.es/m/15150.
1557257111@sss.pgh.pa.us