Add doc block

docs/hooks/hook_civicrm_container.md

@@ -36,8 +36,14 @@ This hook is available in CiviCRM 4.7+.
 ```php
     use Symfony\Component\Config\Resource\FileResource;
     use Symfony\Component\DependencyInjection\Definition;
-
-    function mymodule_civicrm_container($container) {
+    use Symfony\Component\DependencyInjection\ContainerBuilder;
+
+  /**
+   * Add my services to the container.
+   * 
+   * @param \Symfony\Component\DependencyInjection\ContainerBuilder $container
+   */
+    function mymodule_civicrm_container(ContainerBuilder $container) {
       $container->addResource(new FileResource(__FILE__));
       $container->setDefinition('mysvc', new Definition('My\Class', array()))->setPublic(TRUE);
     }