Add cue creation info for WebM for text tracks.
Add Matroska to DataCue mapping info in WebM section for informative
purposes.
Add some more examples of when a in-band track doesn't map to just one
HTML track.
Merge branch 'upstream'

This adds changes from Bob from:
https://github.com/w3c/HTMLSourcingInbandTracks/commit/2af51c751e473a40f50775f2a14e1213f635d649
https://github.com/w3c/HTMLSourcingInbandTracks/commit/69c0208a4a3302cb666ab1455197ae4fdfb4c514
Wording changes to the text track cue mapping section of MPEG-2.
Merge pull request #1 from silviapfeiffer/master

Wording changes to the text track cue mapping section of MPEG-2.