A section describing to the programmers ever object in the game and how it should behave is a good thing to have in a D-doc. Also a section on the players controls what keys do what and how the player is controled, is he able to jump, run, swim?. And if you have specail FX like lights that explode you should detail these in way that the programmers and artist can understand so they don't have to keep trying to realize what is in your mind. For example if you have a rocket launcher and when the rocket hits a object you want there to be 3 flames 2 force wakes and a flash of light then write is as descriptivly as possable. this tells the arts what type of models and sprites to use and the programmer how to make then act. A side from this detail the world, does it have rain in it ? gravity is it earth normal? does the sun always shine or is there a day and night cycle to the game. Detail every weapon in the game. what is its range and damage? does it have a ammo limit? do you have to press a button to reload it? on top of that good concept art is great and storyboards too. hook up with a any 2d artist this is a D-doc not a fine painting so you don't have to have the best art work in the world. have them do sketchs of the world in action and so key objects like weapons and charcters. Write a section on the menus which menu leads to the next and what buttons are on each menu. Give the readed some simple picture to show what it is you are describing. Make a menu flow chart as well.

Just a few Idea.
Malice