I agree with 
Kyatric
That's basically what I do, except I write all of mine is gibberish most of the time, for myself to understand, and to quickly write reminders before I lose my train of thought.
As for conveying ideas to your team members. I have never needed to use a boring black and white document to convey my colorful game idea to my team. To be honest, it's a pretty horrible way to do it. Maybe having the ability to convey ideas through speech is hard for some, but it's really the best way in smaller teams.
Pitching game ideas? You will never need to present your game design document in a 20 minute pitch. You may need one if you're formally submitting your game to a large publisher. But there are still smaller documents these people would want to see.
My suggestion - only do it if you're doing it because it helps you in some way. Don't do it as a formality to make your game feel more concrete. In windows desktops, it takes 5 seconds to jot down your ideas on a sticky note, it takes 2-3 minutes to open up Word, find the relevant sections and write down your idea.