As someone who's been down this road in the past, unless you're already rather familiar with Erlang and OTP's conventions, the gap between the User's Guide and the API documentation is far too large to make connecting information you learn from one with information you learn from the other easy.
I've summarized my personal struggle with the Cowboy user's guide and documentation in my other comment in the thread. Please read that comment before you downvote this one. :)