From the templating chapter of the book:

"In Symfony2, almost every part of a bundle can be overridden so that you can use and customize it for your specific application. "

After searching far and wide, ie. the book, cookbook, forum, google, etc. there does not seem too much meat beyond the brief statement above on how to accomplish this.

The references I know about so far are:

http://symfony.com/doc/2.0/book/templating.html#overriding-bundle-templates
http://symfony.com/doc/current/cookbook/event_dispatcher/class_extension.html
http://symfony.com/doc/current/cookbook/event_dispatcher/method_behavior.html
https://groups.google.com/forum/#!topic/symfony-users/o1hj4tQZB28

In particular, one might need to know how to extend things like: bundle controllers, models, entities, validation rules, routes, config settings, services, service extensions, forms, etc.

Keep up the great work on the documentation. It is really starting to take shape.