client API: clarify pause/unpause events, modify core-idle property

Whether you consider the semantics weird or not depends on your use
case, but I suppose it's a bit confusing anyway. At this point, we keep
MPV_EVENT_PAUSE/UNPAUSE for compatibility only.

Make the "core-idle" property somewhat more useful in this context.
client API: improve mpv_observe_property docs

Document the "normal" behavior (if MPV_FORMAT_NONE is not used) first,
and then introduce MPV_FORMAT_NONE as exception.

The actual semantics didn't change in mpv; this is only clarification.
command: add cache-buffering-state property
player: remove unnecessary code

This part is already done by open_stream_async().