diff --git a/includes/form.inc b/includes/form.inc
index 1b163a0..1ffff24 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -1899,6 +1899,14 @@ function form_builder($form_id, &$element, &$form_state) {
 /**
  * Populate the #value and #name properties of input elements so they
  * can be processed and rendered.
+ *
+ * Each input element may pass an optional #value_callback argument to specify 
+ * a function through which input data is filtered before being passed to the
+ * #value property. If no #value_callback argument is present, a default
+ * callback is attempted based upon the input element's #type property:
+ *   'form_type_' . $element['#type'] . '_value';
+ * Such callbacks are passed three values: $element, $input, and $form_state:
+ *   $element['#value'] = $value_callback($element, $input, $form_state);
  */
 function _form_builder_handle_input_element($form_id, &$element, &$form_state) {
   if (!isset($element['#name'])) {
@@ -2154,7 +2162,7 @@ function form_state_values_clean(&$form_state) {
 /**
  * Helper function to determine the value for an image button form element.
  *
- * @param $form
+ * @param $element
  *   The form element whose value is being populated.
  * @param $input
  *   The incoming input to populate the form element. If this is FALSE,
diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index cc67924..89559cd 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -226,7 +226,12 @@ function hook_cron_queue_info_alter(&$queues) {
  *  - "#submit": array of callback functions taking $form and $form_state.
  *  - "#title_display": optional string indicating if and how #title should be
  *    displayed, see theme_form_element() and theme_form_element_label().
+ *  - "#value_callback": specify a function through which input data is filtered
+ *    before being passed to the #value property. If no #value_callback argument
+ *    is present, a default callback is attempted based upon the input element's
+ *    #type property: 'form_type_' . $element['#type'] . '_value';
  *
+ * @see hook_element_info()
  * @see hook_element_info_alter()
  * @see system_element_info()
  */