mention alterNewInstance in extension doc

Resources/doc/reference/extensions.rst

@@ -3,8 +3,8 @@ Extensions
 
 Admin extensions allow you to add or change features of one or more Admin instances. To create an extension your class
 must implement the interface ``Sonata\AdminBundle\Admin\AdminExtensionInterface`` and be registered as a service. The
-interface defines a number of functions which you can use to customize the edit form, list view, form validation and
-other admin features.
+interface defines a number of functions which you can use to customize the edit form, list view, form validation,
+alter newly created objects and other admin features.
 
 .. code-block:: php