Update and clean up documentation.
Merge branch 'anton/video_subtitles' of github.com:edx/edx-platform into anton/video_subtitles
Merge branch 'anton/video_subtitles' of github.com:edx/edx-platform into anton/video_subtitles
Update tests.
Merge branch 'anton/video_subtitles' of github.com:edx/edx-platform into anton/video_subtitles