docs: eip service annotation - mention subnet discovery

[alebedev87]

Issue

Description

This PR changes a note for service.beta.kubernetes.io/aws-load-balancer-eip-allocations annotation doc by mentioning the subnet discovery (additionally to the subnet annotation). This way the user doesn't have to follow the subnet annotation link to find out that the discovery may also be involved. IMO this makes the note clearer. WDYT?

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 🌟

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: alebedev87
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[k8s-ci-robot]

Hi @alebedev87. Thanks for your PR.

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