Clarify authorisation

[David W Chadwick]

More text should be added to explain the authorisation model more clearly. Who is being authorised by whom? And what role do VPs play in the authorisation? This issue is related to issue 1458 (incomplete swimlanes) as the swimlanes should show the authorisation flow.

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[Kristina Yasuda]

completed in PR #228 - there is a pre-authorized code flow and authorization code flow. users can be asked to present VP as part of authentication as described in## Wallet Initiated Issuance during Presentation (requires presentation of additional Credentials during issuance) {#use-case-2}, but how is out of scope of the protocol.