Add missing documentation for PGC_SU_BACKEND in description of pg_settings.

Add missing documentation for PGC_SU_BACKEND in description of pg_settings.

Commit fe550b2ac249af5fbd8e9e19290a4ba43c882f2d 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.