docs: Explain how to silence overly verbose messages by fop

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20230324194701.dqkzcdtlcikseo22@awork3.anarazel.de

doc/src/sgml/docguide.sgml

@@ -373,6 +373,16 @@ ADDITIONAL_FLAGS='-Xmx1500m'
     swapping or will not work at all.
    </para>
 
+   <para>
+    In its default configuration <productname>FOP</productname> will emit an
+    <literal>INFO</literal> message for each page. The log level can be
+    changed via <filename>~/.foprc</filename>:
+<programlisting>
+LOGCHOICE=-Dorg.apache.commons.logging.Log=org.apache.commons.logging.impl.SimpleLog
+LOGLEVEL=-Dorg.apache.commons.logging.simplelog.defaultlog=WARN
+</programlisting>
+   </para>
+
    <para>
     Other XSL-FO processors can also be used manually, but the automated build
     process only supports FOP.