Document UI events.
Document UI events.

Closes #1870

Change-Id: I17a49dbc6d27923b361eb6ac7dda9a0a34354f43

shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player