I had this with the OpenStack API. It's so hilariously underdocumented that I've reverse-engineered it by poking at a Devstack and checking the HTTP requests in mitmproxy. After some time, even when I had to work with a new part of the API, I didn't even bother to check for documentation because just poking around was faster (and more reliable: one never knows how close the documentation is to the real deal).