diff --git a/core/lib/Drupal/Core/Form/FormInterface.php b/core/lib/Drupal/Core/Form/FormInterface.php
index 2cafbebfa02..4754a9a25ae 100644
--- a/core/lib/Drupal/Core/Form/FormInterface.php
+++ b/core/lib/Drupal/Core/Form/FormInterface.php
@@ -12,6 +12,10 @@ interface FormInterface {
   /**
    * Returns a unique string identifying the form.
    *
+   * The returned ID should be a unique string that can be a valid PHP function
+   * name, since it's used in hook implementation names such as
+   * hook_form_FORM_ID_alter().
+   *
    * @return string
    *   The unique string identifying the form.
    */