Commit fe550b2a missed updating this list
of the PGC_XXX values, which in hindsight is not so surprising because
catalogs.sgml is not a place you'd think to look for them.  In addition to
adding the missing doco, insert the PGC_XXX C enum names in SGML comments,
so that grepping for the enum names will find this file.  That might spare
the next person similar embarrassment.

Spotted by Magnus Hagander.