diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index a7205d1..58752ef 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -38,12 +38,13 @@ public function getFormId($form_arg, &$form_state);
    *
    * @param \Drupal\Core\Form\FormInterface|string $form_arg
    *   A form object to use to build the form, or the unique string identifying
-   *   the desired form. If $form_arg is a string and a function with that
-   *   name exists, it is called to build the form array. Modules that need to
+   *   the desired form. If $form_arg is a string and is the name of a class
+   *   implementing \Drupal\Core\Form\FormInterface then an instance of that
+   *   class is created to build the form.  Otherwise if the string is the name
+   *   of a function, it is called to build the form array. Modules that need to
    *   generate the same form (or very similar forms) using different $form_ids
    *   can implement hook_forms(), which maps different $form_id values to the
-   *   proper form constructor function. Examples may be found in node_forms(),
-   *   and search_forms().
+   *   proper form constructor function.
    * @param ...
    *   Any additional arguments are passed on to the functions called by
    *   drupal_get_form(), including the unique form constructor function. For