Update OAuth Section

[jischr]

Non-normative updates + ascii for clarity

[jischr]

Alternative option to the ascii drawing would be to replace with the text below (or similar). Both accomplish my goal of clarity on the mapping of OAuth roles to SSF Roles. @thomasdarimont what do you think would be most useful to a reader?

## OAuth Support

This profile requires OAuth 2.0 {{RFC6749}} support. The following roles apply:

* Client: the SSF Receiver
* Resource Server: the SSF Transmitter
* Authorization Server: a service trusted by the SSF Transmitter

[thomasdarimont]

I think the classification is sufficient. Perhaps we can find a more precise description for "a service trusted by the SSF Transmitter", e.g.: "Authorization Server: the OAuth 2.0 Authorization Server that issues access tokens accepted by the SSF Transmitter endpoints."