Designs change so much during development and afterwards that most of my carefully crafted documents rot away in source control and become almost more of a hindrance than a help, once code is in production. I see design documents as necessary to good communication and to clarify your thinking while you develop something, but after that it takes a herculean effort to keep them properly maintained.
I do take pictures of whiteboards and save the JPEGs to source control. Those are some of my best design docs!
