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/Database/AbstractModel.php

@@ -44,10 +44,19 @@ abstract class AbstractModel extends Eloquent
      */
     protected $afterDeleteCallbacks = [];
 
+    /**
+     * @internal
+     */
     public static $customRelations = [];
 
+    /**
+     * @internal
+     */
     public static $dateAttributes = [];
 
+    /**
+     * @internal
+     */
     public static $defaults = [];
 
     /**