add some more comments

common/step_provision.go

@@ -23,12 +23,17 @@ import (
 //   <nothing>
 
 func PopulateProvisionHookData(state multistep.StateBag) map[string]interface{} {
-	hookData := map[string]interface{}{}
+	// Load Builder hook data from state, if it has been set.
+	// hookData := state.GetOk("generated_data").(map[string]interface{})
+
 	// instance_id is placed in state by the builders.
 	// Not yet implemented in Chroot, lxc/lxd, Azure, Qemu.
 	// Implemented in most others including digitalOcean (droplet id),
 	// docker (container_id), and clouds which use "server" internally instead
 	// of instance.
+
+	// Also note that Chroot and lxc/lxd builders tend to have their own custom
+	// step_provision, so they won't use this code path.
 	id, ok := state.GetOk("instance_id")
 	if ok {
 		hookData["ID"] = id