Add new order ratio being added to 8.0.0

[terry1purcell]

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What is changed, added or deleted? (Required)

Prior PR added tidb_opt_ordering_index_selectivity_ratio, but this was not documented.

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[ti-chi-bot]

Hi @terry1purcell. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Oreoxmt]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-04-02 08:16:22.560165609 +0000 UTC m=+345444.087706151: ☑️ agreed by hfxsd.
• 2024-04-07 06:46:05.086253388 +0000 UTC m=+772026.613793934: ☑️ agreed by Oreoxmt.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[hfxsd]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #16992.