APIdoc is described as 'Inline Documentation for RESTful web APIs' and is an app in the development category. There are more than 10 alternatives to APIdoc for a variety of platforms, including Self-Hosted solutions, Online / Web-based, Windows, Linux and Mac. The best alternative is swagger.io, which is both free and Open Source. Other great apps like APIdoc are ReDoc, Docma, Aglio and ng-docx.
aglio - An API Blueprint renderer that supports multiple themes and outputs static HTML that can be served by any web host. API Blueprint is a Markdown-based document format that lets you write API descriptions and documentation in a simple and straightforward way.
Building APIs redefined. Speed up development of services, mobile, and web applications with auto-generated client, server, and documentation in many languages. Cut the costs and development time by up to 50%.