Clarify which metrics are only applicable to a particular kind according to implementation interest.

[henbos]

Some metrics are defined for both audio and video but have only been implemented for one of the kinds. As a follow-up to "move unimplemented metrics to provisional spec" (#621 / #622) we should go over the metrics in this document again and for any metrics only implemented for <kind>, update the spec to say it is only applicable for <kind>.

[vr000m]

Related to #689 where we need to fix this for totalProcessingTime