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.