Yes design documents suck the creativity out of people and shit but face it, without writing down and documenting your idea, all you will do is dabble in code for days and get no where.
The things I mentioned above are parts taken from commonly used design documents, the most important parts.
So this would be the name of the game, if you have one. Then write down how you want to make the player feel, this helps you figure out the atmosphere and general theme of your game. Writing down the core game mechanics are a must, without a solid idea you will just be going around in circles.
The other parts such as Time, Scope and Budget are important. Nobody wants to deal with any of this rubbish but at the same time, having a good idea of how long a project will take (or take to get to a state you deem “playable”, at which point you can start getting creative), how big the project is going to be and if you can (if your going to) afford to pay artists/sound engineers.
And a schedule does not really have to be a solid “things must be finished by [insert date here]” kind of thing, but it is nice to have a list of things you want to do in X amount of time, this will give you goals and help you get things done.
The copyright thing, well I use one because I share my ideas with a team of people and not that I don’t trust them but have to protect my own interests.
tl;dr GD’s can kill creativity and are boring to write, however they help you structure your project better until it gets to a stage it is easy add/alter/remove code.