Doc: update VACUUM VERBOSE freezing tip.

VACUUM VERBOSE/autovacuuming logging have reported on the number of
pages frozen by VACUUM since commit d977ffd added that capability.
This information is directly related to relfrozenxid advancement, so
update an older tip from the documentation about how relfrozenxid is
reported on by the same instrumentation code.  Now the tip directly
mentions newly frozen pages, too.

Author: petergeoghegan

doc/src/sgml/maintenance.sgml

@@ -643,8 +643,9 @@ SELECT datname, age(datfrozenxid) FROM pg_database;
      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
+     <structfield>relminmxid</structfield> advanced, and the number of
+     newly frozen pages.  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>