Improve documentation of ContentAwareTrait

Author: endroid

book/controller.rst

@@ -378,11 +378,17 @@ 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`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.
+    :class:`Symfony\\Component\\DependencyInjection\\ContainerAware`. The service
+    container object will then be accessible via the ``container`` property.
+
+.. tip::
+
+    If your host uses PHP 5.4 or higher, you can also use the
+    class:`Symfony\\Component\\DependencyInjection\\ContainerAwareTrait` trait to
+    make the container accessible via the ``container`` property.
+
+.. versionadded:: 2.4
+    The ``ContainerAwareTrait`` is new in Symfony 2.4
 
 .. note::