Add clarity to use of verb metadata

We had a little debate here yesterday about verb metadata. This PR represents my personal view on what's implied as the way verb metadata can and must be handled. I see this as just clarifications, rather than new requirements, but I think that's open to debate.

Let's discuss!
Sadly probably can't be a now requirement, even with the extreme vagueness about what canonical *is*, but I don't think there's anything preventing us from SHOULD*-ing it. And if an LRS does happen to have a canonical representation around, I think it's reasonable to SHOULD* picking out the most appropriate language when a canonical statement fetch is done.