ipn: update comments that refer to Options.Prefs

Things are slightly less tangled now that we've migrated prefs to the
backend (and renamed the field to LegacyMigrationPrefs).

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

ipn/prefs.go

@@ -58,8 +58,9 @@ type Prefs struct {
 	// It would be more consistent to restart controlclient
 	// automatically whenever this variable changes.
 	//
-	// Meanwhile, you have to provide this as part of Options.Prefs or
-	// Options.UpdatePrefs when calling Backend.Start().
+	// Meanwhile, you have to provide this as part of
+	// Options.LegacyMigrationPrefs or Options.UpdatePrefs when
+	// calling Backend.Start().
 	ControlURL string
 
 	// RouteAll specifies whether to accept subnets advertised by