Fixing OAuth documentation on h.readthedocs.io

Problems:

1) Internally, there are different opinions about when we use OAuth vs SSO vs Third-party integration. We should pick one set of terms for each process and use them consistently in conversations within and without the company.

For requests where we provide an OAuth token, users create Hypothesis accounts (authority = hypothes.is), we should call that "OAuth".

For requests where we provide a third-party integration (authority != hypothes.is) we should call that "third-party", or "third-party/SSO".

2) Our documentation doesn't discriminate between OAuth and 3rd-party/SSO. I've added some annotations displaying this confusion [in this group](https://hypothes.is/groups/EYmX5oPm/read-the-docs-notes) here:

- https://hyp.is/X_-_nmMREeq9M1eXnTKHbw/h.readthedocs.io/en/latest/api/authorization/
- https://hyp.is/X_-_nmMREeq9M1eXnTKHbw/h.readthedocs.io/en/latest/api/authorization/

Proposed solutions:

1. Standardize language internally and on h.readthedocs.io, differentiating between OAuth and 3rd-party/SSO.
2. Correct instances where our documentation either references the wrong thing, or comes to an incorrect conclusion.