[Observium] Update http://www.observium.org/docs/add_device/ for missing "search" line with resolv.conf

Hello!

I've wird problem with "Can't resolve ...." when adding device (from ./add_device.php and from website).

Te solution was simple - there was missing "search domain" at /etc/resolv.conf - i don't find this kind of problem and solution, so you can add it at site to Troubleshooting.

Thanks.

Regards.