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!
That's definitely the behaviour I'd like to see for 1.1 and what we should be moving towards.  On this other hand, for this version the format query parameter doesn't mention verb display so we can't require the LRS to return canonical verb displays in that scenario. I can see this MUST is in the wrong direction in that scenario though.

How about:

``` * When queried for statements with a format of "exact", the LRS MUST always return the Display property exactly as included (or omitted) within the Statement. ```

That way this PR doesn't comment on canonical at all. We can then raise a separate issue around how verbs, attachments and extension language maps should be handled in "ids" and "canonical" format.