Issue #3203416 by guilhermevp, joachim: docs for...

Issue #3203416 by guilhermevp, joachim: docs for FormValidator::doValidateForm() should explain $form_id can detect recursion

core/lib/Drupal/Core/Form/FormValidator.php

@@ -223,7 +223,9 @@ protected function finalizeValidation(&$form, FormStateInterface &$form_state, $
    *   not be repeated in the submission step.
    * @param $form_id
    *   A unique string identifying the form for validation, submission,
-   *   theming, and hook_form_alter functions.
+   *   theming, and hook_form_alter functions. Is only present on the initial
+   *   call to the method, which receives the entire form array as the $element,
+   *   and not on recursive calls.
    */
   protected function doValidateForm(&$elements, FormStateInterface &$form_state, $form_id = NULL) {
     // Recurse through all children, sorting the elements so that the order of