docs(security): enhance explanation of peer certificate authentication

[ronaldngounou]

Contributes to Issue #1062

[k8s-ci-robot]

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[k8s-ci-robot]

@ronaldngounou: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-website-lint	0d8fee1	link	true	/test pull-website-lint

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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