Merge branch '3.4' into 4.4

* 3.4:
  [#13000] Fixed newline
  [#12722] Added filename comment to avoid confusion
  Update micro_kernel_trait.rst
  Update broken links

Author: wouterj

configuration/micro_kernel_trait.rst

@@ -22,6 +22,7 @@ via Composer:
 
 Next, create an ``index.php`` file that defines the kernel class and executes it::
 
+    // index.php
     use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
     use Symfony\Component\Config\Loader\LoaderInterface;
     use Symfony\Component\DependencyInjection\ContainerBuilder;