[DOC] add KAT documentation

[ChuksGrinage]

Issue

#3670

Description

Customer requested a feature for adding client keepalive annotations. This feature is already available as every load balancer attribute is and therefore no code changes is needed. This PR is simply a doc update to make it clearer for future reference

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ChuksGrinage / name: Chuks Grinage (1e567b8)

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @ChuksGrinage. Thanks for your PR.

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[oliviassss]

/lgtm
/approve

[shraddhabang]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ChuksGrinage, oliviassss, shraddhabang

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