docs: add built-in PriorityClass values

[iacker]

Summary

• add the built-in numeric values for system-cluster-critical and system-node-critical
• show readers how to inspect those values directly in their cluster with kubectl
• clarify that values above 1 billion are reserved for built-in system PriorityClasses

Validation

• verified against upstream Kubernetes scheduling source (pkg/apis/scheduling/types.go and pkg/apis/scheduling/v1/helpers.go)
• independent content review of the doc change

Fixes #54107

[k8s-ci-robot]

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[kfess]

duplicate of #54386