No documentation that's public-facing, unfortunately. It's only in the iqfeed.net developer section which is only visible to subscribers. However, if you used the IQFeed API in 2016, things have not changed very much, and you can even specify an older protocol if you want. What is available, including a list of what's new, is at
http://www.iqfeed.net/index.cfm?displayaction=developer§ion=main. You may also send any specific questions you have to the IQFeed API Support address, or your sales contact, and I can answer them for you.
Sincerely,
Gary Stephen
DTN IQFeed Implementation Support Specialist