ipn/ipnlocal: update comment about using FallbackResolvers for exit nodes

While reading the DNS code noticed that we were still using FallbackResolvers
in this code path but the comment was out of date.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/local.go

@@ -2769,10 +2769,12 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs ipn.PrefsView, logf logger.
 	case len(dcfg.DefaultResolvers) != 0:
 		// Default resolvers already set.
 	case !prefs.ExitNodeID().IsZero():
-		// When using exit nodes, it's very likely the LAN
-		// resolvers will become unreachable. So, force use of the
-		// fallback resolvers until we implement DNS forwarding to
-		// exit nodes.
+		// When using an exit node, we send all DNS traffic to the exit node, so
+		// we don't need a fallback resolver.
+		//
+		// However, if the exit node is too old to run a DoH DNS proxy, then we
+		// need to use a fallback resolver as it's very likely the LAN resolvers
+		// will become unreachable.
 		//
 		// This is especially important on Apple OSes, where
 		// adding the default route to the tunnel interface makes