doc: complementary to desc of `in-mem-slow-query-topn-num` and and `in-mem-slow-query-recent-num`.

[AilinKid]

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[Oreoxmt]

@AilinKid Could you please invite a tech reviewer? Thanks!

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[qiancai]

Removed the needs-cherry-pick-release-8.3 label because the v8.3 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v8.3 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-8.4 label because the v8.4 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v8.4 and will no longer receive new updates.