[ttml2] tts:disparity text

I just updated the specification text for tts:disparity [1] with
information I pulled from the Sapporo minutes and email threads.

Please comment if any further changes are required.

[1]
https://rawgit.com/w3c/ttml2/master/spec/ttml2.html#style-attribute-disparity

Received on Friday, 4 March 2016 07:24:47 UTC