Hi guys!

First, see symfony/symfony#11881. Basically, we should be using service decoration and not recommending that you override any *class parameters. We do talk about service decoration in components/dependency_injection/advanced.rst, but I propose that we show a quick cookbook article on how to override a real core service.

It might also be nice to change the existing article to use examples better than foo and bar :). See https://github.com/symfony/symfony-docs/pull/3753/files#r17416550.