I used to do design docs … I now use Trello as this is more organic and we can change things as we need in a visual way as well as see who is doing what.
What I do however is add implemented features to a Game Design Document and keep the High Level Overview updated … this allows the members of the team who are not involved with development to be kept in the loop and not feel overwhelmed by the Trello boards.
It also gives me a nice reference document when we are ready for release that I can give to the marketing dude and he can start working his voodoo.