Yes, the end events are set when you stop recording an automation track. If you record again (overdub) onto the same track, the end event will be moved later if you record longer, but will not be moved earlier if you record shorter -- so overdubbing a small section of the automation track won't change the end event.
You can, but don't have to, record automation for the full duration of the song to set an end event. If you don't want to record for the full duration, you can just record until the last automation event, then edit the automation track and manually move the end event to match the full duration.
If you do not set the correct time for the end event, and you are playing a recording, the song completion action still won't be triggered until the recording finishes. In other words, an end event that occurs while a recording is playing will be ignored. So if you want to trigger song completion actions and you're using backing tracks, you can ignore the end events. I think the way I wrote the release notes was misleading, and I'm going to change that now.
The main purpose of the end event is to allow someone to trigger a song completion action without using backing tracks and without relying on the song elapsed time, which starts when the song is selected and doesn't take into account any time spent introducing the song or changing instruments before starting to play. Now the timing of the trigger is more explicit and directly controllable.