A Design Doc is basically the whole planning document of a mod\game
In there you have plot description, the assets (ships, weapons etc) you will need, mission outline and detail, gameplay description (not really needed for a freespace mod, unless you're planning on a brand new gameplay feature or whatever), among others.
The idea of writing this document is to:
1. The designer get his ideas in place, organize them, etc
2. Be compreensive enough so that the other team members, by reading it, understand the whole thing and vision of the mod.
So obviously images, graphs, whatever may help to get the idea across.
Hope that helps
