Well your post is a few months old by now, but on the off-chance you’re still around here’s a reply.
Ken never did get back to me with digital copies of Roberta’s design documents. However, I did see a few that she still had, and basically they are just like a script for a movie, but way longer. Every background is described, every character action and response is described and all possibilities for interacting with objects, other characters or the environment are all planned out.
Now I’m sure different designers had different ways of doing things. Al Lowe, for example, said once that he never wrote the dialogue down for some of his games until he actually typed it into Sierra’s development tool programs.
Anyway, I would recommend you visit Al Lowe’s website AlLowe.com. He’s posted online design documents for some of his Larry games, Freddy Pharkas, and Torin’s Passage. They should give you a good idea about game design and they’re also just really cool to look at.