add language about selective disclosure considerations

[decentralgabe]

fix #285

examples to be subsequently fixed by w3c/respec-vc#41

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[decentralgabe]

@msporny let me know if this addresses your concerns

[dlongley]

Approving, though in my opinion, there could be more nuance here. Some of these properties are clearly required for consistent semantics (e.g., @context, type), others are really "up to the issuer". If the issuer doesn't mind a holder presenting without credential status, for example, I don't think that issuer MUST include it as mandatory. Who knows what the credential status is for here (e.g., it may not be for revocation).

So I don't think this language has to be as strict as it is, but since I'm not volunteering to come up with alternative language either, I approve as-is.

[TallTed]

Does this do what you were thinking appropriate, @dlongley?

[dlongley]

@TallTed, yes, I can also accept SHOULD and leave the nuance to the issuer/implementer. I'd accept MUST for @context and type as well, but SHOULD is ok for all of them and I think is better for the other properties, IMO.

[decentralgabe]

Open for over a week. Changes accepted. Editorial approval. Merging!