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.
"A user agent may also decide to expose tracks coded in formats it does not support.."
That makes no sense. A UA can't expose a track that it can't at least parse, which is the first step in decoding.