I think Koa (or Express) are the best tools for developing an API spec/mock/doc that's useable and useful from the get go. You can have an endpoint working in seconds, and it's easy to add all the different scenarios you need. Dropping in middleware is easy too. And if you write your code well, it's self documenting.

And ultimately, it's documentation for developers. I think it's as easy for a non-js dev to parse and understand an endpoint controller than it is to parse whatever freaky-deaky API documentation someone has cobbled together.