Deprecate internal constants (#96)

Author: derrabus

src/Doctrine/Instantiator/Instantiator.php

@@ -28,8 +28,12 @@ final class Instantiator implements InstantiatorInterface
      * Markers used internally by PHP to define whether {@see \unserialize} should invoke
      * the method {@see \Serializable::unserialize()} when dealing with classes implementing
      * the {@see \Serializable} interface.
+     *
+     * @deprecated This constant will be private in 2.0
      */
-    public const SERIALIZATION_FORMAT_USE_UNSERIALIZER   = 'C';
+    public const SERIALIZATION_FORMAT_USE_UNSERIALIZER = 'C';
+
+    /** @deprecated This constant will be private in 2.0 */
     public const SERIALIZATION_FORMAT_AVOID_UNSERIALIZER = 'O';
 
     /**