A series of tweets were released by BioWare’s Joe Hendriks recently and he stated in them how hard it was that it was to come up with missions and other documentation from scratch. He said that getting feedback on the design or how a mission works is a key part of the process.
Writing out designs for missions and how the mechanics will work was part of the next tweet and the fact that more than one person is working on mission development makes documentation even more important to the overall development of the game.
Hendriks goes on to state that each mission has a writer, cinematic designer, artist, audio artist, level designer and a QA analysts that are working on it all at the same time. Keeping all of those people on track and any new designer that comes on track only comes from great documentation.