It’s almost like Tenable is trying to hide the documentation, almost every link on the Internet takes you to their documentation for the Tenable Cloud service!
Either way to get to the REST API documentation go to /api path, if your server was at https://scanner.myworldofit.net:8443 you’d go to https://scanner.myworldofit.net:8443/api.
Of note you can go to /api#/resources/session/create to enter the username/password and then use the built-in tester capability of the WebUI.