diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index a5690df..da8b328 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1902,7 +1902,10 @@ function theme_breadcrumb($variables) {
  * @endcode
  *
  * @param array $element
- *   A structured array containing two sub-levels of elements.
+ *   A structured array containing two sub-levels of elements. Properties used:
+ *   - #tabledrag: The value is a list of arrays that are passed to
+ *     drupal_add_tabledrag(). The HTML ID of the table is prepended to each set
+ *     of arguments.
  *
  * @see system_element_info()
  * @see theme_table()
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index df9e464..9687296 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -572,6 +572,10 @@ function system_element_info() {
     '#process' => array('form_process_table'),
     '#element_validate' => array('form_validate_table'),
     // Properties for tabledrag support.
+    // The value is a list of arrays that are passed to drupal_add_tabledrag().
+    // drupal_pre_render_table() prepends the HTML ID of the table to each set
+    // of arguments.
+    // @see drupal_add_tabledrag()
     '#tabledrag' => array(),
     // Render properties.
     '#pre_render' => array('drupal_pre_render_table'),