Make document more readable and fix captionservice link.

Mostly inserts paragraph tags around <li> content.
Fix typo s/TextTrackCue/TextTrackCue/
Clarify the possibilities of exposing in-band MPEG-2 SCTE 27 subtitles.
Clarify how CEA708 captions are exposed in MPEG-2.
Merge pull request #9 from silviapfeiffer/mpeg2DataCue

Mpeg2 data cue
Merge pull request #8 from silviapfeiffer/spacing

Make document more readable and fix captionservice link.