minor #9948 Document the required ClassMetadataFactory (deviantintegral, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #9948).

Discussion
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Document the required ClassMetadataFactory

Symfony PR at symfony/symfony#26534

Commits
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bc011c0 Reword
b05c1cc Codefence ClassMetadataFactory
a4cb67a Document the required ClassMetadataFactory

Author: javiereguiluz

components/serializer.rst

@@ -169,10 +169,11 @@ needs three parameters:
     Support for the ``allow_extra_attributes`` key in the context was introduced
     in Symfony 3.3.
 
-By default, additional attributes that are not mapped to the denormalized
-object will be ignored by the Serializer component. Set the ``allow_extra_attributes``
-key of the deserialization context to ``false`` to let the serializer throw
-an exception when additional attributes are passed::
+By default, additional attributes that are not mapped to the denormalized object
+will be ignored by the Serializer component. If you prefer to throw an exception
+when this happens, set the ``allow_extra_attributes`` context option to
+``false`` and provide an object that implements ``ClassMetadataFactoryInterface`
+when constructing the normalizer::
 
     $data = <<<EOF
     <person>
@@ -184,6 +185,9 @@ an exception when additional attributes are passed::
 
     // this will throw a Symfony\Component\Serializer\Exception\ExtraAttributesException
     // because "city" is not an attribute of the Person class
+    $classMetadataFactory = new ClassMetadataFactory(new AnnotationLoader(new AnnotationReader()));
+    $normalizer = new ObjectNormalizer($classMetadataFactory);
+    $serializer = new Serializer(array($normalizer));
     $person = $serializer->deserialize($data, 'Acme\Person', 'xml', array(
         'allow_extra_attributes' => false,
     ));