Document ad events.
Document ad events.

Issue #2469.

Change-Id: I3a301bdd22377f5d79eeae97eb8caa186518bb81

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