OCPBUGS-568: Add --dest-ignition admonition for custom CA certs

Author: kowen-rh

modules/installation-user-infra-machines-advanced-customizing-live-ca-certs.adoc

@@ -32,6 +32,12 @@ $ coreos-installer pxe customize rhcos-<version>-live-initramfs.x86_64.img \
 ----
 endif::[]
 +
+[IMPORTANT]
+====
+The `coreos.inst.ignition_url` kernel parameter does not work with the `--ignition-ca` flag.
+You must use the `--dest-ignition` flag to create a customized image for each cluster.
+====
++
 [NOTE]
 ====
 Custom CA certificates affect how Ignition fetches remote resources but they do not affect the certificates installed onto the system.