SDP-US Finished

I've just posted an update at [1]. This incorporates fixes for all issues
I'm aware of, both technical and editorial. I also made a final pass fixing
a number of random typos, poor wording, etc. So it should be good to go
unless new issues are raised.

[1] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml10-sdp-us/Overview.html