This will be a tracking bug.
What's the hg URL of said "mozilla-webmedia repository"? Full source seems to be here (but not terribly useful as patch): <http://dm-download02.mozilla.org/pub/mozilla.org/firefox/releases/devpreview/1.9.3a4webm/source/> Builds: <http://nightly.mozilla.org/webm/>
It's a local repo. We won't be publishing it directly.
Neither this nor any of its dependencies appear to actually impact developer documentation, except the addition of the YASM dependency for builds, which is already documented. Removing doc needed keyword.