diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index 334e10b..ca79437 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -137,9 +137,9 @@ public function rebuildForm($form_id, FormStateInterface &$form_state, $old_form
    * by calling $form_state->getErrors().
    *
    * @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.
+   *   The value must be one of the following:
+   *   - The name of a class that implements \Drupal\Core\Form\FormInterface.
+   *   - An instance of a class that implements \Drupal\Core\Form\FormInterface.
    * @param $form_state
    *   The current state of the form. Most important is the
    *   $form_state->getValues() collection, a tree of data used to simulate the