minor tweak to catalogs.sgml pg_class.reltablespace

While reviewing the patch for SET ACCESS METHOD[1] I noticed that
pg_class.relam is not documented fully for partitioned tables, so I
proposed the attached.  Also, I remove a comment that merely repeats
what was already said a few lines above.

This is intended for backpatch to 12.

[1] https://postgr.es/m/202403191013.w2kr7wqlamqz@alvherre.pgsql

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/