[release-2.5] Adding more explicit documentation regarding the node's ready label.

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #1200

/assign ybettan

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ybettan]

/close

This will be cherry-picked together with all missing commits in #1204

[k8s-ci-robot]

@ybettan: Closed this PR.

In response to this:

/close

This will be cherry-picked together with all missing commits in #1204

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