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.
Updated docs to note override for config

  * calling configureHandlers and configureListeners overrides the defaults due to boolean checks to see if handlers/listeners are configured.
Merge pull request #1 from ReservedDeveloper/feature/closureDocumentationUpdate

Updated docs for Closure configuration params