Add comment regarding tcclass.guarantee

Shorewall/Perl/Shorewall/Tc.pm

@@ -188,6 +188,7 @@ our $sticky;
 #              occurs    => <number> # 0 means that this is a class generated by another class with occurs > 1
 #              parent    => <class number>
 #              leaf      => 0|1
+#              guarantee => <sum of rates of sub-classes>
 #              options   => { tos  => [ <value1> , <value2> , ... ];
 #                             tcp_ack => 1 ,
 #                             ...