Backend cleanup (#2859)

* Extender docblocks cleanup
* Excplicit type hinting in extenders
* Bring method under constructor
* Mark some classes and methods as internal
* Remove beta references

Co-authored-by: Clark Winkelmann <clark.winkelmann@gmail.com>

src/Notification/MailableInterface.php

@@ -27,7 +27,5 @@ interface MailableInterface
      *
      * @return string
      */
-    // Uncomment beta 17. Commented as temporary BC layer since Symfony changed
-    // the namespace of their translator interface
-    // public function getEmailSubject(TranslatorInterface $translator);
+    public function getEmailSubject(TranslatorInterface $translator);
 }

src/Notification/NotificationSyncer.php

@@ -166,8 +166,10 @@ class NotificationSyncer
      *
      * @param string $driverName
      * @param NotificationDriverInterface $driver
+     *
+     * @internal
      */
-    public static function addNotificationDriver(string $driverName, NotificationDriverInterface $driver)
+    public static function addNotificationDriver(string $driverName, NotificationDriverInterface $driver): void
     {
         static::$notificationDrivers[$driverName] = $driver;
     }
@@ -182,6 +184,8 @@ class NotificationSyncer
 
     /**
      * @param callable|string $callback
+     *
+     * @internal
      */
     public static function beforeSending($callback): void
     {