include more details about the kubeadm upgrade backup process

[neolit123]

during upgrade if the process fails on static pods, kubeadm writes a folder called /etc/kubernetes/backup.

currently we do not:

• document how this folder works. WIP
• have test coverage when kubeadm falls back to a manifest in the backup folder. EDIT: we have some unit tests for this.

xref: #1617 (comment)

[Klaven]

I will document this and start working on the test coverage.

[neolit123]

adding help-wanted to see if someone wants to take this one.
/help

[k8s-ci-robot]

@neolit123:
This request has been marked as needing help from a contributor.

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In response to this:

adding help-wanted to see if someone wants to take this one.
/help

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[neolit123]

a PR for this is here:
https://github.com/kubernetes/website/pull/18424/files