update config example

Signed-off-by: Kristoffer Dalby <[email protected]>

config-example.yaml

@@ -242,6 +242,7 @@ dns:
   magic_dns: true
 
   # Defines the base domain to create the hostnames for MagicDNS.
+  # This domain _must_ be different from the server_url domain.
   # `base_domain` must be a FQDN, without the trailing dot.
   # The FQDN of the hosts will be
   # `hostname.base_domain` (e.g., _myhost.example.com_).