diff --git a/core/lib/Drupal.php b/core/lib/Drupal.php
index e95e80f558..1786c7f9bf 100644
--- a/core/lib/Drupal.php
+++ b/core/lib/Drupal.php
@@ -102,7 +102,16 @@ class Drupal {
   protected static $container;
 
   /**
-   * The currently active container object, or NULL if not initialized yet.
+   * The LegacyMessenger instance.
+   *
+   * Note: this is merely used to ensure that the instance survives when
+   * \Drupal::messenger() is invoked. It is required to ensure that messages
+   * are properly transferred to the proper Messenger service once the
+   * container is initialized. Do not store the Messenger service here.
+   *
+   * @todo Remove once LegacyMessenger has been removed before 9.0.0.
+   *
+   * @see https://www.drupal.org/node/2928994
    *
    * @var \Drupal\Core\Messenger\LegacyMessenger|null
    */