Adding notes about Extension registration to the DI component

components/dependency_injection/compilation.rst

@@ -119,6 +119,12 @@ processed when the container is compiled at which point the Extensions are loade
     // ...
     $container->compile();
 
+.. note::
+
+    When loading a config file that uses an extension alias as a key, the
+    extension must already have been registered with the container builder
+    or an exception will be thrown.
+
 The values from those sections of the config files are passed into the first
 argument of the ``load`` method of the extension::
 
@@ -239,6 +245,27 @@ but also load a secondary one only if a certain parameter is set::
         }
     }
 
+.. note::
+
+    Just registering an extension with the container is not enough to get
+    it included in the processed extensions when the container is compiled.
+    Loading config which uses the extension's alias as a key as in the above
+    examples will ensure it is loaded. The container builder can also be
+    told to load it with its
+    :method:`Symfony\\Component\\DependencyInjection\\ContainerBuilder::loadFromExtension`
+    method::
+
+        use Symfony\Component\DependencyInjection\ContainerBuilder;
+        use Symfony\Component\Config\FileLocator;
+        use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
+
+        $container = new ContainerBuilder();
+        $extension = new AcmeDemoExtension;
+        $container->registerExtension($extension);
+        $container->loadFromExtension($extension->getAlias());
+        $container->compile();
+
+
 .. note::
 
     If you need to manipulate the configuration loaded by an extension then