more documentation for navigate away pompt js util

closes #377

static/js/utils/form.js

@@ -308,6 +308,8 @@
     }
 
     function beforeUnloadHandler(event) {
+      // allow the event to happen if the form was not touched by the
+      // user or the unload event was initiated by a form submit
       if (!touched || unloadDueToSubmit) {
         return false;
       }