[docs] add more uses cases to node-selector service annotation

[kayrus]

What this PR does / why we need it:

Add more context about the node-selector service annotation and how to use it in combination with the node.kubernetes.io/exclude-from-external-load-balancers node label.

Which issue this PR fixes(if applicable):
fixes #

Special notes for reviewers:

See also #2601

Release note:

NONE

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[kayrus]

/remove-lifecycle stale