At the beginning of the Game Design Document a clear and succinct description of the game is vital, it gives the reader (whether they be an stakeholder or a team member) a really efficient and to the point blueprint of the what the game will be.
Abstract for Raptured (Working title):
Raptured is a story driven puzzle game where players act as an Angel sent to Earth to bring the souls of humans to their afterlife. The Angel collects souls that float above piles of clothes of raptured humans, these souls add substance to the Angel and allows it to move objects in the world. To reach the next souls darkness must be removed by moving objects to shine light on it. Then Angels objective is to find as many souls as possible and reach the highest point of the level and ascend back to the skies.
Any questions and feedback welcome, this is still very early in the process and the idea may change so keep coming back to see the development.