[kapep]

I think they mean "test code is documentation". For example if there's a unit test that expects an error for a certain input, it serves as documentation that this kind of input is not allowed.

It not always feasible to document every little edge case in natural language and keep it in sync with your code. If you "document" edge cases as tests, they _have_ to be in sync with your code. It shouldn't replace traditional documentation though and is better suited for internal components and not for public API.