Update docs around slow_query tables

[dveeden]

What is changed, added or deleted? (Required)

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For details, see tips for choosing the affected versions.

• [x] master (the latest development version)
• [ ] v8.4 (TiDB 8.4 versions)
• [ ] v8.3 (TiDB 8.3 versions)
• [ ] v8.2 (TiDB 8.2 versions)
• [ ] v8.1 (TiDB 8.1 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions)

What is the related PR or file link(s)?

• This PR is translated from:

• Other reference link(s):

• https://github.com/pingcap/tidb/pull/20200

• https://github.com/pingcap/tidb/issues/56707

Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
• [ ] Need modification after applied to another branch
• [ ] Might cause conflicts after applied to another branch

[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-10-18 03:04:00.763969115 +0000 UTC m=+585837.912878947: :ballot_box_with_check: agreed by crazycs520.
• 2024-10-22 08:41:17.633532508 +0000 UTC m=+340478.330323113: :ballot_box_with_check: agreed by qiancai.
• 2024-10-23 07:07:56.303240725 +0000 UTC m=+421277.000031334: :ballot_box_with_check: agreed by hfxsd.

[qiancai]

/approve

[ti-chi-bot[bot]]

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