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!
Maybe, but do we want to require the LRS to maintain canonical verbs? That's got to be a 1.1 thing, right?