What do I mean by living documentation? As the application is built, automated tests are continually being written to ensure that specific criteria are met. These tests come in many different shapes and sizes, ranging from unit tests to integration tests, and leading up to end-to-end functional tests and beyond. All of these tests describe, in some way, how the application works. You have to admit that this sounds just like documentation.
This documentation may not be perfect, but that doesn't stop it from being documentation. Think of an application that has some unit tests, and maybe one badly written end-to-end test. I would equate that to the sort of documentation that you would get with a cheap electrical product, from somewhere such as China. It comes with a manual, which will undoubtedly have a small badly written English bit that doesn...
