Update docs around slow_query tables

[dveeden]

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

• This PR is translated from:

• Other reference link(s):

• executor: tiny optimize slow_query performance and add related runtime stats tidb#20200

• RuntimeStats for cluster_slow_query  tidb#56707

Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[qiancai]

Rest LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

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

[qiancai]

/approve

[ti-chi-bot]

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