Merge branch '3.4' into 4.2

* 3.4:
  fix (serializer): invert xml and yaml bloc descriptions

Author: javiereguiluz

components/serializer.rst

@@ -254,14 +254,14 @@ for each format:
 
     $classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
 
-* XML files::
+* YAML files::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\YamlFileLoader;
 
     $classMetadataFactory = new ClassMetadataFactory(new YamlFileLoader('/path/to/your/definition.yaml'));
 
-* YAML files::
+* XML files::
 
     use Symfony\Component\Serializer\Mapping\Factory\ClassMetadataFactory;
     use Symfony\Component\Serializer\Mapping\Loader\XmlFileLoader;