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/Formatter/Formatter.php

@@ -44,21 +44,33 @@ class Formatter
         $this->cacheDir = $cacheDir;
     }
 
+    /**
+     * @internal
+     */
     public function addConfigurationCallback($callback)
     {
         $this->configurationCallbacks[] = $callback;
     }
 
+    /**
+     * @internal
+     */
     public function addParsingCallback($callback)
     {
         $this->parsingCallbacks[] = $callback;
     }
 
+    /**
+     * @internal
+     */
     public function addUnparsingCallback($callback)
     {
         $this->unparsingCallbacks[] = $callback;
     }
 
+    /**
+     * @internal
+     */
     public function addRenderingCallback($callback)
     {
         $this->renderingCallbacks[] = $callback;