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

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ff6783ce526031fe9134f4d2139a78f9fd52d3d5

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)