doc: Mention the connstring key word for PGSERVICE

The documentation for the connection service file was mentioning
the environment variable early but not the connection string key
word until the last sentence and only then in an example.  This
adds the keyword in the first paragraph to make it clearer

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/87r09ibpke.fsf@wibble.ilmari.org

doc/src/sgml/libpq.sgml

@@ -9318,7 +9318,8 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
   <para>
    The connection service file allows libpq connection parameters to be
    associated with a single service name. That service name can then be
-   specified in a libpq connection string, and the associated settings will be
+   specified using the <literal>service</literal> key word
+   in a libpq connection string, and the associated settings will be
    used. This allows connection parameters to be modified without requiring
    a recompile of the libpq-using application. The service name can also be
    specified using the <envar>PGSERVICE</envar> environment variable.