DOCS: kill the rest of the old mplayer tech documentation

Not really useful anymore, other than confusing everyone.
ao: document some functions
client API: wait for remaining asynchronous requests before terminating

Sending an asynchronous request and then calling mpv_destroy() would
crash the player when trying to send the reply to the removed client.
Fix this by waiting until all remaining replies have been sent.
client API: rename MPV_EVENT_PLAYBACK_START, add MPV_EVENT_SEEK

Rename MPV_EVENT_PLAYBACK_START to MPV_EVENT_FILE_LOADED.

Add MPV_EVENT_SEEK and MPV_EVENT_PLAYBACK_RESTART.