pgsql: Refer to pg_ident.conf as config file for username mapping, as

Log Message:
-----------
Refer to pg_ident.conf as config file for username mapping, as it's
now used for other things than just ident authentication.

Noted by Stephen Frost

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.276 -> r1.277)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.276&r2=1.277)