Not sure about documentation, but you will need a Messaging Server configured on Policy Manager under Administration. Then go through the Service Templates under Configuration to build the Guest with MAC authentication services. The template will configure a bandwidth and session limit Enforcement Profiles, which you can then go in and edit as necessary. One note - The bandwidth is not capping them at a certain speed, it's saying "no data beyond this amount".
Then go into Guest and create a guest registration page and enable SMS delivery of receipt. Make sure on the form you have phone number as a field, as that will be the default field for sending the SMS.