E2e: Document self hosted machine templates

The self hosted test has optional variables that can be used to switch
the infrastructure machine template when doing the upgrade. This
documents the variables and also explains that the templates will need
to be labeled in order to move with the cluster.

test/e2e/self_hosted.go

@@ -59,6 +59,10 @@ type SelfHostedSpecInput struct {
 	// If true, the variable KUBERNETES_VERSION is expected to be set.
 	// If false, the variables KUBERNETES_VERSION_UPGRADE_FROM, KUBERNETES_VERSION_UPGRADE_TO,
 	// ETCD_VERSION_UPGRADE_TO and COREDNS_VERSION_UPGRADE_TO are expected to be set.
+	// There are also (optional) variables CONTROL_PLANE_MACHINE_TEMPLATE_UPGRADE_TO and
+	// WORKERS_MACHINE_TEMPLATE_UPGRADE_TO for switch the infrastructure machine template
+	// during the upgrade. Note that these templates need to have the clusterctl.cluster.x-k8s.io/move
+	// label in order to be moved to the self hosted cluster (since they are not part of the owner chain).
 	SkipUpgrade bool
 
 	// ControlPlaneMachineCount is used in `config cluster` to configure the count of the control plane machines used in the test.