[User Guide] Rate-Limit annotations lack explanation of confusing behavior

[tback]

User Guide Rate-Limiting Annotations are easily understood as global limits when they're in fact per ingress controller replica. Users in need of exact rate limits cannot use the horizontal autoscaler anymore. Users do not necessarily know the number of deployed replicas of the ingress controller.

The rate limiting section therefore needs at least an in detail explanation of this rather confusing behavior.

[k8s-ci-robot]

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[strongjz]

@tback, Can you update the documentation? This feature was removed, so I'm not hard-pressed to put it on the Maintainers' TODO list.

#11851