kernel-netlink: Don't install routes for CHILD_SAs with interface ID

src/swanctl/swanctl.opt

@@ -934,6 +934,8 @@ connections.<conn>.children.<child>.if_id_out = 0
 	instance, beyond that the value _%unique-dir_ assigns a different unique
 	interface ID for each CHILD_SA direction (in/out).
 
+	The daemon will not install routes for CHILD_SAs that have this option set.
+
 connections.<conn>.children.<child>.set_mark_in = 0/0x00000000
 	Netfilter mark applied to packets after the inbound IPsec SA processed them.