Add language around "Amend" actions

[msporny]

The use cases should speak to "amending" a verifiable credential... specifically, how this might be different from "disputing" a credential, and how an amendment might be added by the original issuer (using VCs), a different issuer (using VCs), or timelines extended via digital signatures (using embedded or external proofs).

This issue came out of a discussion here (note the WG discussion on the topic): #124

[KDean-GS1]

GS1 may have a use case for this, which we will submit shortly.