auto-increment: modify the description about the case of non-continuous auto increment id

[tiancaiamao]

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pingcap/tidb#53315

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[LGTM Timeline notifier]

Timeline:

• 2024-05-22 03:01:23.666063415 +0000 UTC m=+2227037.423198988: ☑️ agreed by qiancai.
• 2024-05-23 01:42:39.931796439 +0000 UTC m=+2308713.688932013: ☑️ agreed by Oreoxmt.

[qiancai]

/approve

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[qiancai]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17433.