[Serializer] add documentation for serializer callbacks

related https://github.com/symfony/symfony/issues/10229

                                       
| Q             | A                   |
| ---           | ---                 |
| Doc fix?      | no                  |
| New docs?     | yes                 |
| Applies to    | 2.3+                |
| Fixed tickets | self :baby:      |
| License       | CC-ASA 3.0 Unported |

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