system tables: add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE

[qiancai]

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Add a note for DATA_LENGTH, INDEX_LENGTH, and TABLE_SIZE to explain they are logical estimates instead of actual physical sizes.

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[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[dveeden]

@mjonss I assume that partitioning and global index work as expected when it comes to the storage size explained here?

[dveeden]

TIKV_REGION_STATUS seems to depend on Prometheus being available. However this isn't noted on https://docs.pingcap.com/tidb/stable/information-schema-tikv-region-status

The result is that size calculation would fail with this error:

ERROR 1105 (HY000): query metric error: [domain:9009]Prometheus address is not set in PD and etcd

If a custom Prometheus is used then the address might not be configured correctly in pd/etcd. If there is no Prometheus we may want to see if there are any alternatives available

[ti-chi-bot[bot]]

@AilinKid: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to [this](https://github.com/pingcap/docs/pull/19391#pullrequestreview-2435127044): > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.