docs: document kubelet_cmd_flags field and precedence behavior

[Copilot]

Addresses documentation feedback from #7771 for the new kubelet_cmd_flags field that enables deprecation of the map-based kubelet_flags.

Changes

• Function documentation: Added comment to getKubeletFlags() explaining precedence behavior—when kubelet_cmd_flags is present, it's returned as-is; otherwise falls back to sorted key-value pairs from kubelet_flags map

• Proto README: Updated KubeletConfig section documenting that KubeletCmdFlags accepts pre-formatted string flags and takes precedence over KubeletFlags

// getKubeletFlags returns the kubelet command-line flags as a string.
// When kubelet_cmd_flags is present, it takes precedence and is returned as-is (raw string).
// Otherwise, it falls back to constructing flags from the kubelet_flags map (sorted key-value pairs).
func getKubeletFlags(kubeletConfig *aksnodeconfigv1.KubeletConfig) string {
	if kubeletConfig.GetKubeletCmdFlags() != "" {
		return kubeletConfig.GetKubeletCmdFlags()
	}
	return createSortedKeyValuePairs(kubeletConfig.GetKubeletFlags(), " ")
}

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