Update APC part of Performance page

I think somebody should update the APC part of the Performance page here:
http://symfony.com/doc/current/performance.html

I think we should add information about APCu and the Symfony Component polyfill-apcu there.
https://github.com/symfony/polyfill-apcu

If you think this is a good idea - feel free to work on it. I don't have time for it - sorry!