I think the best approach here is really going to depend on your tools and the kind of game you’re writing, but for my part I’ve tended to write up general notes that break down the overall act structure (which for a parser game, often corresponds to the geography), puzzle concepts, plus snatches of dialogue or descriptive details that I want to get across once I get into implementation – basically getting enough down to know where each chunk of game is headed and how the whole piece fits together, without duplicating too much effort when it comes time to move to coding.
If it’s helpful, I linked to the final version of my design doc in the post-mortem I wrote for my first game – you can download it if you scroll all the way to the bottom of this post.