This is only a tiny bit tangential, but I'd be interested to see the initial design doc for XCOM: Enemy Unknown, which went through several major design overhauls prior to release. As I understand it, this kind of thing is not at all uncommon: i.e. adherence to a design doc is secondary to making a fun and complete game.
I had thought that the point behind a design doc was simply to get people interested in funding and working on a project, as well as to lay out the ground work to start development; during development a lot of stuff is going to change and you'll probably find the game will diverge quite a bit from the ideas laid out in that document. Really the difference comes down to this: when you call the design doc done and when you start really building your game.
Here's a very good read by GD's own Tom Sloper regarding design documents. Link.