diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 1fb9c06..6b2e73b 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -2387,7 +2387,8 @@ protected function assertNoTitle($title, $message = '', $group = 'Other') {
    * @param $message
    *   (optional) A message to display with the assertion. Do not translate
    *   messages: use format_string() to embed variables in the message text, not
-   *   t(). If left blank, a default message will be displayed.
+   *   t(). Using '%callback' in the message will add 'theme_[callback]()' to
+   *   the message. If left blank, a default message will be displayed.
    *
    * @return
    *   TRUE on pass, FALSE on fail.