sql-statement-explain: add explanation related to scalar subquery

[qiancai]

What is changed, added or deleted? (Required)

As the title says.

Which TiDB version(s) do your changes apply to? (Required)

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By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• 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)
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• 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: sql-statement-explain: add explanation related to scalar subquery docs-cn#18727
• Other reference link(s):

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

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-09-30 10:59:12.457069581 +0000 UTC m=+266107.877282587: ☑️ agreed by hfxsd.

[qiancai]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19026.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.2: #19027.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #19028.