statistics: Improve iteration example

Clarify a hidden gotcha: the stat name iterator does not include the
stat group!

modules/statistics/doc/statistics_admin.xml

@@ -246,9 +246,11 @@ stat_iter_init("packet", "iter");
 		<title><function>stat_iter_next</function> usage</title>
 		<programlisting format="linespecific">
 ...
-stat_iter_init("packet", "iter");
-while (stat_iter_next($var(k), $var(v), "iter")) {
-	...
+# periodically clear packet-related data
+timer_route [clear_packet_stats, 7200] {
+	stat_iter_init("packet", "iter");
+	while (stat_iter_next($var(stat), $var(val), "iter"))
+		reset_stat("packet:$var(stat)");
 }
 ...
 </programlisting>