feature symfony#3701 [Serializer] add documentation for serializer callbacks (cordoval)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes symfony#3701).

Discussion
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[Serializer] add documentation for serializer callbacks

related symfony/symfony#10229

| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | no                  |
| New docs?     | yes                 |
| Applies to    | 2.3+                |
| Fixed tickets | self 👶      |
| License       | CC-ASA 3.0 Unported |

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Commits
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b865b40 add comma madness and lowercasing W
efe2029 plug new revision
eabdbd0 add serializer set callback documentation

Author: weaverryan

components/serializer.rst

@@ -181,6 +181,37 @@ method on the normalizer definition::
 As a final result, the deserializer uses the ``first_name`` attribute as if
 it were ``firstName`` and uses the ``getFirstName`` and ``setFirstName`` methods.
 
+Using Callbacks to Serialize Properties with Object Instances
+-------------------------------------------------------------
+
+When serializing, you can set a callback to format a specific object property::
+
+    use Acme\Person;
+    use Symfony\Component\Serializer\Encoder\JsonEncoder;
+    use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
+    use Symfony\Component\Serializer\Serializer;
+
+    $encoder = new JsonEncoder();
+    $normalizer = new GetSetMethodNormalizer();
+
+    $callback = function ($dateTime) {
+        return $dateTime instanceof \DateTime
+            ? $dateTime->format(\DateTime::ISO8601)
+            : '';
+    }
+
+    $normalizer->setCallbacks(array('createdAt' => $callback));
+
+    $serializer = new Serializer(array($normalizer), array($encoder));
+
+    $person = new Person();
+    $person->setName('cordoval');
+    $person->setAge(34);
+    $person->setCreatedAt(new \DateTime('now'));
+
+    $serializer->serialize($person, 'json');
+    // Output: {"name":"cordoval", "age": 34, "createdAt": "2014-03-22T09:43:12-0500"}
+
 JMSSerializer
 -------------