DOC: enhance docstring

src/core/js/form_panel.js

@@ -66,6 +66,10 @@ var form_panel = new function () {
 
     /**
      * Creates a callback function that toggles the form panel which
+     *
+     * You may supply 'undefined' as form_config and use form_creator
+     * instead which is supposed to be a function without argument that
+     * return the form to be added to the panel
      */
     this.create_show_form_callback = function (
         panel_id, title, form_config, form_creator=undefined