I can tell you that a game design document is a living document. It evolves with the project and acts as a source of guidance and truth in the present moment. I personally couldn't see the point in "creating" the whole game in the document before actually making the game. The document is initially very simple and rough and is guided by the project goals as you run into design challenges spread through all skills (level design, gameplay, sound design, music, art, vfx, programming, etc) related to the project.