[mediacapture-transform] Pull Request: Remove remaining "audio" references in spec

alvestrand has just submitted a new pull request for https://github.com/w3c/mediacapture-transform:

== Remove remaining "audio" references in spec ==
The ones that still remain are notes saying that exclusion of audio
does not have consensus, and a reference to the previous version API.

Adds an "audio-explainer.md" document to preserve audio info.

Fixes #71

See https://github.com/w3c/mediacapture-transform/pull/72


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