From November 2016 until November 2017, one of my main focuses was improving the API documentation at Keen IO. I ended up giving a talk a few months ago at APIStrat about the things I wish I had known before November 2016.
(Sadly, I do not have audio of the talk.)
Some people in the Write the Docs Slack found it an interesting example of what developers (like me) would like to know before diving into writing docs.
Hopefully, others can find it helpful too! Let me know if you have any questions or feedback.