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!
@fugu13 You raised this before on an earlier diff: https://github.com/adlnet/xAPI-Spec/pull/529#discussion_r20595720 and my response: https://github.com/adlnet/xAPI-Spec/pull/529#discussion_r20661270 .