Added documentation to cover PR #6951 on serializer component.

.. versionadded:: 2.3
     That PR covers PR #6951, enables camelCase formater on deserialize process , that way attributes as attribute_name could be implemented as attributteName parameter, with getAttributeName and setAttributeName, giving different formating option from setAttribute_name getAttribute_name.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#6951)
| Applies to    |  2.3
| Fixed tickets | no ticket