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!
Yes, and I think it's very important. I'll lay out my reasoning:

1. We're introducing variant behavior for language maps in verbs vs other things if we include this change

2. We're forbidding LRSs from performing localization on a piece of data which is there for the sole point of enabling localization, which we not just allow, but require, LRSs to perform localization on elsewhere

3. As an example, we make it so, when someone makes a request to the LRS and says "I want just one language version of stuff, and the language I want is Cantonese if you have it", the LRS puts the Cantonese versions it knows about in a bunch of different places (where the Statement originally had the English version)... except if the verb was originally sent in English, they're getting English, and aren't allowed to see the Cantonese version, even if the LRS knows it.

Regarding your earlier response, if a reporting system wants to possibly use the versions of language maps in the original versions of Statements, it should request the Statements with "exact". The entire point of canonical is that the system doing the pulling wants the best matching language out of all the LRS knows about, regardless of what was sent in the statements, and that's the behavior for every other language map.