tailcfg, tsdns: derive root domains from list of nodes (#708)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns/config.go

@@ -23,6 +23,7 @@ type Config struct {
 	// if the manager does not support per-domain settings.
 	PerDomain bool
 	// Proxied indicates whether DNS requests are proxied through a tsdns.Resolver.
+	// This enables Magic DNS.
 	Proxied bool
 }