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 don't think you need to mention the "rather than" clause in this requirement as the next MAY requirement covers such usage.  I'd cut it off at "Statement" (upper case plz :) )