Thank you for calling attention to this. You're seeing a known bug with our developer documentation. The Community API v1 and v2 sections are auto-generated and pull from comments/annotations in our code. The registration_data resource is internal at this time, and you should not be able to click through to it from the User resource documentation. You will a subset of registration_data as a subobject in queries to the Users collection, but the registration_data resource is not something you can query directly at this time.
