docs: explain retention time for event allocation policy in more detail

docs: explain retention time for event allocation policy in more detail

'when the calendar event' triggers was too vague, it could mean for
the current media-set or the next time. Apart from that, it was not
technically correct all the time, since we take the start time of
the next media set if that exists first.

The idea here is that we begin the retention when the media set is
finished.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>