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/Post/Post.php

@@ -210,6 +210,8 @@ class Post extends AbstractModel
      * @param string $type The post type.
      * @param string $model The class name of the model for that type.
      * @return void
+     *
+     * @internal
      */
     public static function setModel(string $type, string $model)
     {