Follow up of bug OCPBUGS-29657.

As clarified in comment [1] and the replies to that comment, while the procedure described in [2] can be applied to IPI installations, it requires adding the new disk manually to the control plane machines after creating it (it's only possible to automatically add them in AWS IPI clusters by OCPCLOUD-817), so manual steps are required for IPI installations.

A note should be added to [2] to clarify that, as if customer is using the controlplanemachinesets, the new disk will not be automatically added (but in AWS) and it will be needed to add the new disk manually to each control plane node replaced.

[1] https://issues.redhat.com/browse/OCPBUGS-29657?focusedId=24632417&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-24632417
[2] https://docs.openshift.com/container-platform/4.12/scalability_and_performance/recommended-performance-scale-practices/recommended-etcd-practices.html#move-etcd-different-disk_recommended-etcd-practices