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
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6f7d016e917e7960e791bd2fb9d7c3d53f08b9a4

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)