Improve rsync handler documentation
See "thread":https://lists.riseup.net/www/arc/backupninja/2012-04/msg00002.html:
* Total lack of high-level documentation wrt. its general behavior. I'm afraid this is still true, and should be reported as a grave bug on our bug tracker.
(from redmine: created on 2012-04-16, closed on 2012-05-13)