TT#39607 update CF source set mode API docref

* improved the CF "mode" field tooltip
    * the tooltip also states that the "mode" field is mandatory

Change-Id: I44a0bee7ca75d71d0c60df2b45ea9d968e1e54fa

lib/NGCP/Panel/Form/CallForward/CFSourceSetSubAPI.pm

@@ -24,7 +24,7 @@ has_field 'mode' => (
     required => 1,
     element_attr => {
         rel => ['tooltip'],
-        title => ['The source set mode. A blacklist forwards everything except numbers in the list, a whitelist only forwards numbers in this list.']
+        title => ['Source set mode. If set to "blacklist" it enables forwarding for everything except numbers in the list, and "whitelist" only enables forwards for numbers defined in this list. This field is mandatory.']
     },
 );