Nothing about it in docs.

Here are the only links I've found:

http://test.ical.ly/2011/03/24/creating-a-filter-as-a-twig-extension-in-a-symfony2-bundle/
http://www.martinsikora.com/how-to-make-twig-extension-for-symfony2
http://fossplanet.com/f6/%5Bsymfony-users%5D-mini-howto-implement-twig-extension-bundle-insymfony-2-a-93002/