Fix capacity reservation group identity and rollout guidance

[Alex Huang (wenhug)]

Summary

• Correct the user-assigned identity resource ID, principal ID, RBAC scope, and existing-cluster update command.
• Clarify that existing VMSS node pools use a rolling reimage and document the maxSurge and maxUnavailable capacity tradeoffs.
• Document the Virtual Machine Scale Sets limitation, system node pool behavior, and the follow-up required for zero-size reservations.

Validation

• npx --yes markdownlint-cli2 articles/aks/use-capacity-reservation-groups.md
• npx --yes markdown-link-check articles/aks/use-capacity-reservation-groups.md --quiet
• Verified the updated Azure CLI parameters against command help.

[prmerger-automator]

Alex Huang (@wenhug) : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[prmerger-automator]

Alex Huang (@wenhug) : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit c3e54f8:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/aks/use-capacity-reservation-groups.md	✅Succeeded

For more details, please refer to the build report.

[prmerger-automator]

PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.

[Regan Downer (v-regandowner)]

Erin Schaffer (@schaffererin)

Can you review the proposed changes?

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#label:"aq-pr-triaged"
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