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.

Author: petere

doc/src/sgml/libpq.sgml

@@ -6843,7 +6843,7 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
    at <filename>~/.pg_service.conf</filename> or the location
    specified by the environment variable <envar>PGSERVICEFILE</envar>,
    or it can be a system-wide file
-   at <filename>/etc/pg_service.conf</filename> or in the directory
+   at <filename>`pg_config --sysconfdir`/pg_service.conf</filename> or in the directory
    specified by the environment variable
    <envar>PGSYSCONFDIR</envar>.  If service definitions with the same
    name exist in the user and the system file, the user file takes