"API Documentation Best Practices" is a brand new short and free course by APISEC University which looks into why documenting your APIs should be an essential part of your software development cycle and not an afterthought. https://www.i-programmer.info/news/87-web-development/16579-learn-to-document-your-api-correctly.html
A dairy of my work.Just links to the full artices on i-programmer.info