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!
I'm fine with it, though I think it's already required by the language around "exact", so we probably don't need an additional requirement. How about a SHOULD* for adjusting the language map with canonical?