OCPBUGS-59852: Updated how to add control planes to BM Postinstall docs

[dfitzmau]

Version(s):
4.12+

Issue:
OCPBUGS-59852

Link to docs preview:

• SME has approved this change.
• QE has approved this change.

[openshift-ci-robot]

@dfitzmau: This pull request references Jira Issue OCPBUGS-59852, which is invalid:

• expected the bug to target the "4.21.0" version, but no target version was set

Comment /jira refresh to re-evaluate validity if changes to the Jira bug are made, or edit the title of this pull request to link to a different bug.

The bug has been updated to refer to the pull request using the external bug tracker.

In response to this:

Version(s):
4.12+

Issue:
OCPBUGS-59852

Link to docs preview:

• SME has approved this change.
• QE has approved this change.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[ocpdocs-previewbot]

🤖 Tue Sep 02 14:16:07 - Prow CI generated the docs preview:

https://98342--ocpdocs-pr.netlify.app/
https://98342--ocpdocs-pr.netlify.app/openshift-enterprise/latest/post_installation_configuration/cluster-tasks.html

[openshift-ci]

@dfitzmau: all tests passed!

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