Descriptions tracks are enabled by their own Control Bar menu button, and shown like other text tracks.
Display of a text track is marked as 'aria-live', so it will be read by a screen reader when it changes.
A descriptions track cannot be displayed at the same time as a Subtitles or Captions track; if either of
those tracks is enabled, then the Descriptions track is disabled, and the menu button is also disabled.

…k in the sandbox as descriptions.html.example.

Track examples are in docs/examples/elephantsdream.
Descriptions text track from @silviapfeiffer,
Captions from https://github.com/walsh9/videojs-transcript/tree/master/captions (converted from SRT to WebVTT)
Chapters text track created by @OwenEdwards.

…on being disabled when captions are showing.