Updated docs for Closure configuration params

  * Added note to protect Closure via [Pimple::protect](https://github.com/fabpot/Pimple/blob/1.1/lib/Pimple.php#L142]). Without this, the Closure will error out, having been passed the Silex application as a parameter instead of an instance of ```\JMS\Serializer\Handler\HandlerRegistry```.
  * See [Pimple::offsetGet()](https://github.com/fabpot/Pimple/blob/1.1/lib/Pimple.php#L83) for more information.
  * Note: This configuration will change with the introduction of Pimple v3.0, a current dependency in Silex master branch.