Doc: clarify description of regexp fields in pg_ident.conf.

Doc: clarify description of regexp fields in pg_ident.conf.

The grammar was a little shaky and confusing here, so word-smith it
a bit.  Also, adjust the comments in pg_ident.conf.sample to use the
same terminology as the SGML docs, in particular "DATABASE-USERNAME"
not "PG-USERNAME".

Back-patch appropriate subsets.  I did not risk changing
pg_ident.conf.sample in released branches, but it still seems OK
to change it in v18.

Reported-by: Alexey Shishkin <alexey.shishkin@enterprisedb.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/175206279327.3157504.12519088928605422253@wrigleys.postgresql.org
Backpatch-through: 13