doc: Improve claim about location of pg_service.conf
doc: Improve claim about location of pg_service.conf

The previous wording claimed that the file was always in /etc, but of
course this varies with the installation layout.  Write instead that it
can be found via `pg_config --sysconfdir`.  Even though this is still
somewhat incorrect because it doesn't account of moved installations, it
at least conveys that the location depends on the installation.

50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb