As described in Game Objects and Art and Sound Assets, your design doc needs to link to your complete and detailed asset lists defining every object, visual, and audio content used in the game.
You should begin building these even during the prototyping phase, and have complete lists prior to entering production. Knowing from the start the size of this list will give you a good idea of the scope of the game, and thus how long it will take to produce. While these lists will change during production -- and these docs and spreadsheets must be kept up to date -- putting off creating them is a recipe for spiraling scope and ultimate failure.