Index: src/wp-admin/update.php
===================================================================
--- src/wp-admin/update.php	(revision 25946)
+++ src/wp-admin/update.php	(working copy)
@@ -252,6 +252,14 @@
 		include(ABSPATH . 'wp-admin/admin-footer.php');

 	} else {
+		/**
+		 * Fires if a custom action is requested.
+		 *
+		 * The dynamic part of the hook name, $action,
+		 * refers to the action derived from the GET or POST request.
+		 *
+		 * @since 2.8.0
+		 */
 		do_action('update-custom_' . $action);
 	}
 }