The community decides and because @weaverryan is our native friend, he's the one that usually takes the final decision on the language standards. :)

Both versions are correct, but we want a documentation that is consistent. A documentation that looks like it's created by one person, but that has hunderts people behind it. So we have created [documentation standards](http://symfony.com/doc/current/contributing/documentation/standards.html), one of the language standards:

> Do not use [Serial (Oxford) Commas](http://en.wikipedia.org/wiki/Serial_comma);