diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index b995551..25cd4da 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -211,6 +211,8 @@ function callback_queue_worker($queue_item_data) {
  *  - "#submit": array of callback functions taking $form and $form_state.
  *  - "#title_display": optional string indicating if and how #title should be
  *    displayed, see the form-element template and theme_form_element_label().
+ *  - "#value_callback": string indicating the callback function that indicates
+ *     how input is mapped in an element's #value property.
  *
  * @see hook_element_info_alter()
  * @see system_element_info()