[WIP] sql: add `flashback cluster` doc

[Defined2014]

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• [ ] v6.3 (TiDB 6.3 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Oreoxmt
• shichun-0415

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[shichun-0415]

Hi @Defined2014. Since this feature is expected to be released in v6.4, do we have an FD jira issue for it? I didn't find one in the v6.4 FD list.

[Defined2014]

Hi @Defined2014. Since this feature is expected to be released in v6.4, do we have an FD jira issue for it? I didn't find one in the v6.4 FD list.

@shichun-0415 The FD number is 943.

[Oreoxmt]

@bb7133 @JmPotato PTAL again

[Oreoxmt]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4e33413b0d2cee67c0a500afcb94e2adda08bf33