MCO add info on upgrade ack for updated boot images

modules/mco-update-boot-images-disable.adoc

@@ -9,6 +9,22 @@
 
 To disable the updated boot image feature, edit the `MachineConfiguration` object so that the `machineManagers` field is an empty array.
 
+[NOTE]
+====
+If you are updating a {gcp-first} or {aws-first} cluster from {product-title} 4.18 to 4.19, and you have not configured the `managedBootImages` parameter, the update is blocked with a _This cluster is GCP or AWS but lacks a boot image configuration._ message. The update is blocked intentionally on {gcp-short} or {aws-short} clusters in order to alert you that the default updated boot image behavior is changing between version 4.18 and 4.19 to enable updated boot images by default on those platforms .
+
+To allow the update, perform one of the following tasks:
+
+* If you want to allow the feature to be enabled, acknowledge that you are aware of the change in default behavior by patching the `admin-acks` config map by using the following command:
++
+[source,terminal]
+----
+$ oc -n openshift-config patch cm admin-acks --patch '{"data":{"ack-4.18-boot-image-opt-out-in-4.19":"true"}}' --type=merge
+----
+
+* If you do not want the updated boot image feature enabled, explicitly disable the feature by using the following procedure.
+====
+
 If you disable this feature after some nodes have been created with the new boot image version, any existing nodes retain their current boot image. Turning off this feature does not rollback the nodes or machine sets to the originally-installed boot image. The machine sets retain the boot image version that was present when the feature was enabled and is not updated again when the cluster is upgraded to a new {product-title} version in the future.
 
 .Procedure