I used apiary.io which is the best tool I've found for writing API specs. You can write your specs using an online API Blueprint format editor via their site. Alternatively, you can put a text file in your repo and authorize apiary to pull the file every time you make a commit to github. This way you do everything via your development environment.

The documentation they provide based on the specs looks very well designed and intuitive.

One of my favorite features about apiary was the mock backend server they use based on my API specs. So if you're doing a single page app, just write your specs then point your frontend app to the mock server they provide. This greatly simplified development process for me.