Clarifications on which tracks are exposed

This PR corresponds to https://www.w3.org/Bugs/Public/show_bug.cgi?id=26923
If agreeable, I'd like to make a pass on the whole spec to use normative statements (shall, must, ...) so that we can clearly check conformance on the spec.
Nigel, you said: "A user agent may also decide to expose the existence of tracks coded in formats it does not support."

I disagree. If it can't do anything with it, why would it expose its existence? It may not even be able to decide what that actually is that it found.

It's less than worthwhile to say "hmm, I think I found some extra data in this video file, but really I don't know what it is or what you can do with it, but hey, here it is just in case you wanted to know - oh, and I'm marking it as metadata, you know, simply because I have no clue what it is - could be captions or text descriptions for all I know, or maybe it's this new track thing that I heard about where somebody invented a game that they serialised into a track - anyway - gotta move on - enjoy!"

A browser would rather shut up about such a track than to mention it to the Web developer - and that's the polite thing to do.