Documentation: Minor addition to the reference

A result of the following conversation on IRC;

13:20:33 < tv> what options does backend.set_health take as its state parameter?
13:21:38 < tv> I've set a backend as sick, done some maintenance, then I set it back to healthy ... but all the other backends has state 'probe' ... which I got a feeling is what I want; but probe is an invalid parameter
13:21:52 < tv> I take it as I've overridden the probe state with my own, now
13:22:22 <@Mithrandir> probe is called auto, I think
13:22:39 < tv> you are right, thanks :)
13:23:03  * tv looked up in https://www.varnish-cache.org/docs/trunk/reference/varnish-cli.html, but didn't find it.