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!
What I mean here is that if the the verb id represents 'passed', an activity stream couldn't render that as 'failed', but could render it as 'successfully passed' or 'passed the '. Do you agree?