diff --git a/core/lib/Drupal/Core/Cache/CacheableDependencyInterface.php b/core/lib/Drupal/Core/Cache/CacheableDependencyInterface.php
index cbce6aac6a4..e19ad24f30a 100644
--- a/core/lib/Drupal/Core/Cache/CacheableDependencyInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheableDependencyInterface.php
@@ -46,6 +46,8 @@ interface CacheableDependencyInterface {
    *
    * @return int
    *   The maximum time in seconds that this object may be cached.
+   *   An object may be cached permanently by returning
+   *   \Drupal\Core\Cache\Cache::PERMANENT.
    */
   public function getCacheMaxAge();