Document mixing SetOuput* methods with command line flags

[dims]

Found during review:
kubernetes/kubernetes#81164 (comment)

[fejta-bot]

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/lifecycle stale

[fejta-bot]

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/lifecycle rotten

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[k8s-ci-robot]

@fejta-bot: Closing this issue.

In response to this:

Rotten issues close after 30d of inactivity.
Reopen the issue with /reopen.
Mark the issue as fresh with /remove-lifecycle rotten.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/close

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