API documentation is for the REST API (Application Programming Interface) provided by the various OpenStack services. The majority of the doc work is for the API reference.
Refer to API Complete Reference Pages (HTML) for a list of the source files for our current way of documenting APIs, which is WADL (Web Application Description Language).
We are moving towards a new way of documenting the API Reference with this specification in progress: https://review.openstack.org/#/c/177934/
If you are interested in API documentation, please contact AnneGentle.