I don't know of any tools (though others might!), but one of the nice things about JSON:API-compliant APIs is that they all work the same way for tasks like sorting, filtering, etc etc. So, all an individual API really has to document is its different models, urls, and perhaps things like which optional parts of the spec it supports.
Therefore, what I've done in my personal projects is just document the models etc (in a human readable table) and then point users to the JSON API specification and examples page (or to this forum) for details on using all the other features.
That's not a perfect solution, since the spec can still be a bit hard for newcomers to penetrate (help with that would be very much appreciated!), but it works ok, and will only become a better option as more people get familiar with JSON API.
Hope that helps at least a bit!
P.S. Here's the template I use for documenting all my models. It's reasonably pretty and I could spin it out into a standalone project (with better documentation about the input data format) if there's demand.