Document possibility to use ContainerAwareTrait

In addition to symfony/symfony#8488

Author: endroid

book/controller.rst

@@ -378,8 +378,11 @@ itself.
 
     Extending the base class is *optional* in Symfony; it contains useful
     shortcuts but nothing mandatory. You can also extend
-    :class:`Symfony\\Component\\DependencyInjection\\ContainerAware`. The service
-    container object will then be accessible via the ``container`` property.
+    :class:`Symfony\\Component\\DependencyInjection\\ContainerAware`or use the
+    :class:`Symfony\\Component\\DependencyInjection\\ContainerAwareTrait` (>= PHP 5.4).
+    Both provide an implementation to the
+    :class:`Symfony\\Component\\DependencyInjection\\ContainerAwareInterface`, making the
+    service container object accessible via the ``container`` property.
 
 .. note::