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 see how this can be a MUST. In general the LRS must return what was sent "statements are immutable", however we've previously determined that this concept applies to the value or meaning of properties within statements and not any particular serialization (eg: it's valid to change timestamp format as long as it still refers to the same point in time).

It's presumably based on this principle that the following SHOULD* requirements for modifying the display are OK -- however that same principle implies that there was never any implied "MUST" for the exact behavior above -- by these changes we're saying that according to the current spec the LRS can currently return anything it wants for verb display properties and we're filling in the ambiguity. Therefore we can't have any requirement associated with verb display stronger than a "SHOULD*"