Improvements regarding DC API

[awoie]

This PR makes editorial improvements on DC API. Also made the text more consistent with DCQL.

Fixes #453
Fixes #437
Fixes #528
Fixes #541

[jogu]

Note that I added state to the list of possible DC API parameters since it was not clear whether or not is was supported. By reading the current text, it looked like it was, so I added it explicitly.

state serves no useful purpose with the DC API and there's no need for wallets to support it.

Otherwise the following seemed to be a bit contradicting:

Additional request parameters MAY be defined and used with OpenID4VP over the DC API.
The Wallet MUST ignore any unrecognized parameters.

This text is intended to say "Other OAuth parameters must not be used unless they have explicitly been described somewhere as being suitable to use over the DC API." (along with the standard "If you get something you don't understand, ignore it.")

@jogu @Sakurann @danielfett Is state unrecognized in this case and therefore the verifier shouldn't expect the state to come back?

Yes, correct.

[tlodderstedt]

suggested small changes

[Sakurann]

i think this also fixes #541 by separating security considerations section?

[martijnharing]

If the proposed change is merged (see comment), I can approve.

[awoie]

If the proposed change is merged (see comment), I can approve.

done