diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index 0b32f3a8bf9f14d1efc75c54ae4f7a6916fcec8d..c92d0e081c7eb9af1742c79968d79252d3af06cc 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -22,7 +22,10 @@
  * Properties:
  * - #machine_name: An associative array containing:
  *   - exists: A callable to invoke for checking whether a submitted machine
- *     name value already exists. The submitted value is passed as an argument.
+ *     name value already exists. The arguments passed to the callback will be:
+ *     - The submitted value.
+ *     - The element array.
+ *     - The form state object.
  *     In most cases, an existing API or menu argument loader function can be
  *     re-used. The callback is only invoked if the submitted value differs from
  *     the element's #default_value.