Designing Web Design Documentation

As an occasionally competent software developer, I love good documentation. It explains not only how things work but why they work the way they do. At its best, documentation is much more than a guide. It is a statement of principles and best practices, giving people the information they need to not just understand but believe.
As soft skills go in tech land, maintaining documentation is right up there. Smashing has previously explored design documents in a proposal context, but what happens onc