@dbu it's part of Fabian's [Sphinx extensions](https://github.com/fabpot/sphinx-php). See [this section](http://symfony.com/doc/current/contributing/documentation/format.html#adding-links) for more information about API and PHP.net link rules.

I don't know where to cmf api is placed, if it's in api.symfony.com, you can use this rule too. If it's not, you need to create your own extensions and ask Fabien to install and use those for the symfony cmf docs aswell.