07d, in preparation for a rev 07 IETF I-D submission. Total changes (if we recall correctly) for rev 07; none are breaking:

- Updated token introspection spec citation and details.
- Updated OAuth threat model citation.
- Enhanced security considerations section.
- Broaden from successful access = 200 OK to = 2xx Success.
- Explain that the PAT implicitly gives the "subject" of a requested permission.
- Fix resource_set_registration_endpoint keyword mention.