Docs: Custom Shared Backend Security Groups

[tucktuck9]

Updates the Backend Security Groups configuration documentation to clarify important requirements for using Custom Shared Backend Security Groups. This update helps reduce confusion by making these requirements explicit.

Issue

N/A

Description

Added important notes under the Backend Security Groups Configuration section to specify that:

• The Custom Shared Backend Security Group (--backend-security-group option) requires automatic addition of Inbound Rules to Node/ENI Security Group to be enabled
• When using Custom Frontend Security Groups, the annotation service.beta.kubernetes.io/aws-load-balancer-manage-backend-security-group-rules: "true" must be set

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: tucktuck9 / name: Leah Tucker (2992016)

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @tucktuck9. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[zac-nixon]

/lgtm

Thank you!