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.

greenplum-db/gpdb,50wu/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,adam8157/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,oberstet/postgres-xl,greenplum-db/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,techdragon/Postgres-XL,techdragon/Postgres-XL,jmcatamney/gpdb,adam8157/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,lisakowen/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,pavanvd/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,adam8157/gpdb,zeroae/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ovr/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,pavanvd/postgres-xl,adam8157/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,yazun/postgres-xl,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ovr/postgres-xl,ashwinstar/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,zeroae/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,lisakowen/gpdb,ovr/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,ovr/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,oberstet/postgres-xl,adam8157/gpdb