diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 0aa30a9..0fd8acb 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1944,6 +1944,8 @@ function hook_xmlrpc_alter(&$methods) {
  *     WATCHDOG_DEBUG    Debug: debug-level messages
  *   - link: an optional link provided by the module that called the watchdog() function.
  *   - message: The text of the message to be logged.
+ *   - variables: Array of variables to replace in the message on display or NULL 
+ *     if message is already translated or not possible to translate.
  */
 function hook_watchdog(array $log_entry) {
   global $base_url, $language_interface;