Doc: Add relfrozenxid Tip to XID wraparound section.

VACUUM VERBOSE and autovacuum log reports were taught to report the
details of how VACUUM advanced relfrozenxid (and relminmxid) by commit
872770f.  Highlight this by adding a "Tip" to the documentation, next
to related discussion of age(relfrozenxid) monitoring.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wzk0C1O-MKkOrj4YAfsGRru2=cA2VQpqM-9R1HNuG3nFaQ@mail.gmail.com

Author: petergeoghegan

doc/src/sgml/maintenance.sgml

@@ -611,6 +611,19 @@ SELECT datname, age(datfrozenxid) FROM pg_database;
     cutoff XID to the current transaction's XID.
    </para>
 
+   <tip>
+    <para>
+     When the <command>VACUUM</command> command's <literal>VERBOSE</literal>
+     parameter is specified, <command>VACUUM</command> prints various
+     statistics about the table.  This includes information about how
+     <structfield>relfrozenxid</structfield> and
+     <structfield>relminmxid</structfield> advanced.  The same details appear
+     in the server log when autovacuum logging (controlled by <xref
+      linkend="guc-log-autovacuum-min-duration"/>) reports on a
+     <command>VACUUM</command> operation executed by autovacuum.
+    </para>
+   </tip>
+
    <para>
     <command>VACUUM</command> normally only scans pages that have been modified
     since the last vacuum, but <structfield>relfrozenxid</structfield> can only be