diff --git a/core/lib/Drupal/Core/Messenger/LegacyMessenger.php b/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
index 69c229286e..71cfaec09f 100644
--- a/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
@@ -70,6 +70,54 @@ public function deleteByType($type) {
   }
 
   /**
+   * Sets a message to display to the user.
+   *
+   * Messages are stored in a session variable and displayed in the page template
+   * via the $messages theme variable.
+   *
+   * Example usage:
+   * @code
+   * drupal_set_message(t('An error occurred and processing did not complete.'), 'error');
+   * @endcode
+   *
+   * @param string|\Drupal\Component\Render\MarkupInterface $message
+   *   (optional) The translated message to be displayed to the user. For
+   *   consistency with other messages, it should begin with a capital letter and
+   *   end with a period.
+   * @param string $type
+   *   (optional) The message's type. Defaults to 'status'. These values are
+   *   supported:
+   *   - 'status'
+   *   - 'warning'
+   *   - 'error'
+   * @param bool $repeat
+   *   (optional) If this is FALSE and the message is already set, then the
+   *   message won't be repeated. Defaults to FALSE.
+   *
+   * @return array|null
+   *   A multidimensional array with keys corresponding to the set message types.
+   *   The indexed array values of each contain the set messages for that type,
+   *   and each message is an associative array with the following format:
+   *   - safe: Boolean indicating whether the message string has been marked as
+   *     safe. Non-safe strings will be escaped automatically.
+   *   - message: The message string.
+   *   So, the following is an example of the full return array structure:
+   *   @code
+   *     array(
+   *       'status' => array(
+   *         array(
+   *           'safe' => TRUE,
+   *           'message' => 'A <em>safe</em> markup string.',
+   *         ),
+   *         array(
+   *           'safe' => FALSE,
+   *           'message' => "$arbitrary_user_input to escape.",
+   *         ),
+   *       ),
+   *     );
+   *   @endcode
+   *   If there are no messages set, the function returns NULL.
+   *
    * @internal 
    */
   private function setMessage($message = NULL, $type = 'status', $repeat = FALSE) {
@@ -123,9 +171,6 @@ private function setMessage($message = NULL, $type = 'status', $repeat = FALSE)
    * @see status-messages.html.twig
    *
    * @internal
-   * @deprecated Deprecated as of Drupal 8.5.
-   *   Use \Drupal::service('messenger')->getMessages() or
-   *   \Drupal::service('messenger')->getMessagesByType() instead.
    */
   private function getMessages($type = NULL, $clear_queue = TRUE) {
     if ($messages = $this->setMessage()) {