Add a versionadded directive

Author: javiereguiluz

service_container/service_decoration.rst

@@ -126,6 +126,10 @@ but keeps a reference of the old one as ``.inner``:
                 ->decorate(Mailer::class);
         };
 
+.. versionadded:: 6.1
+
+    The ``#[AsDecorator]`` attribute was introduced in Symfony 6.1.
+
 The ``decorates`` option tells the container that the ``App\DecoratingMailer``
 service replaces the ``App\Mailer`` service. If you're using the
 :ref:`default services.yaml configuration <service-container-services-load-example>`,