diff --git a/core/lib/Drupal/Core/Messenger/MessengerInterface.php b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
index fcc5b55..bd0f705 100644
--- a/core/lib/Drupal/Core/Messenger/MessengerInterface.php
+++ b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
@@ -3,27 +3,26 @@
 namespace Drupal\Core\Messenger;
 
 /**
- * Stores messages send out to the user on the page.
+ * Stores runtime messages sent out to individual users on the page.
  *
- * Examples for these messages are for example that a specific content got
- * saved.
+ * An example for these messages is for example: "Content X got saved".
  */
 interface MessengerInterface {
 
   /**
    * A status message.
    */
-  const STATUS = 'status';
+  const TYPE_STATUS = 'status';
 
   /**
    * A warning.
    */
-  const WARNING = 'warning';
+  const TYPE_WARNING = 'warning';
 
   /**
    * An error.
    */
-  const ERROR = 'error';
+  const TYPE_ERROR = 'error';
 
   /**
    * Adds a new message to the queue.
@@ -33,22 +32,23 @@
    *   consistency with other messages, it should begin with a capital letter
    *   and end with a period.
    * @param string $type
-   *   (optional) The message's type. Either self::STATUS, self::WARNING, or
-   *   self::ERROR.
+   *   (optional) The message's type. Either self::TYPE_STATUS,
+   *   self::TYPE_WARNING, or self::TYPE_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 $this
    */
-  public function addMessage($message, $type = self::STATUS, $repeat = FALSE);
+  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE);
 
   /**
    * Gets all messages.
    *
    * @return array[]
    *   Keys are message types and values are indexed arrays of messages. Message
-   *   types are either self::STATUS, self::WARNING, or self::ERROR.
+   *   types are either self::TYPE_STATUS, self::TYPE_WARNING, or
+   *   self::TYPE_ERROR.
    */
   public function getMessages();
 
@@ -56,7 +56,8 @@ public function getMessages();
    * Gets all messages of a certain type.
    *
    * @param string $type
-   *   The messages' type. Either self::STATUS, self::WARNING, or self::ERROR.
+   *   The messages' type. Either self::TYPE_STATUS, self::TYPE_WARNING,
+   *   or self::TYPE_ERROR.
    *
    * @return string[]
    */
@@ -73,7 +74,8 @@ public function deleteMessages();
    * Deletes all messages of a certain type.
    *
    * @param string $type
-   *   The messages' type. Either self::STATUS, self::WARNING, or self::ERROR.
+   *   The messages' type. Either self::TYPE_STATUS, self::TYPE_WARNING, or
+   *   self::TYPE_ERROR.
    *
    * @return $this
    */
diff --git a/core/lib/Drupal/Core/Messenger/SessionMessenger.php b/core/lib/Drupal/Core/Messenger/SessionMessenger.php
index e293fbd..fc858fd 100644
--- a/core/lib/Drupal/Core/Messenger/SessionMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/SessionMessenger.php
@@ -8,9 +8,6 @@
 
 /**
  * Provides a session-based messenger.
- *
- * This is the default implementation used by Drupal to store message to the
- * user.
  */
 class SessionMessenger implements MessengerInterface {
 
@@ -35,7 +32,7 @@ public function __construct(KillSwitch $page_cache_kill_switch) {
   /**
    * {@inheritdoc}
    */
-  public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
+  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
     if (!isset($_SESSION['messages'][$type])) {
       $_SESSION['messages'][$type] = [];
     }
@@ -46,7 +43,7 @@ public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
     }
 
     // Do not use strict type checking so that equivalent string and
-    // MarkupInterface objects are detected.
+    // \Drupal\Core\Render\Markup objects are detected.
     if ($repeat || !in_array($message, $_SESSION['messages'][$type])) {
       $_SESSION['messages'][$type][] = $message;
       $this->pageCacheKillSwitch->trigger();
diff --git a/core/lib/Drupal/Core/Messenger/StaticMessenger.php b/core/lib/Drupal/Core/Messenger/StaticMessenger.php
index 52f7dcb..f2e0f51 100644
--- a/core/lib/Drupal/Core/Messenger/StaticMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/StaticMessenger.php
@@ -15,8 +15,8 @@ class StaticMessenger implements MessengerInterface {
    * The messages that have been set.
    *
    * @var array[]
-   *   Keys are either self::STATUS, self::WARNING, or self::ERROR. Values are
-   *   arrays of arrays with the following keys:
+   *   Keys are either self::TYPE_STATUS, self::TYPE_WARNING, or
+   *   self::TYPE_ERROR. Values are arrays of arrays with the following keys:
    *   - message (string): the message.
    *   - safe (bool): whether the message is marked as safe markup.
    */
@@ -42,7 +42,7 @@ public function __construct(KillSwitch $page_cache_kill_switch) {
   /**
    * {@inheritdoc}
    */
-  public function addMessage($message, $type = self::STATUS, $repeat = FALSE) {
+  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
     if (!isset($this->messages[$type])) {
       $this->messages[$type] = [];
     }