I prefer a hybrid approach. A document explaining some common things to do and generally the idea behind why the API is structured that way (shows me you actually thought about it, and makes it more logical to find different parts of it without necessarily looking it up), and then an API spec showing all the parameters.
Maybe this makes me old, but I much prefer a written document explaining how an API works over Swagger.
I prefer a hybrid approach. A document explaining some common things to do and generally the idea behind why the API is structured that way (shows me you actually thought about it, and makes it more logical to find different parts of it without necessarily looking it up), and then an API spec showing all the parameters.