Change the description for max_execution_time.

[Benjamin2037]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

我们已经修改了 max_execution_time 的作用范围，也在文档中做相应的修改。

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

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

• master (the latest development version)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
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• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: *: skip log timeout on maxExecutiontime as slow query for DDL statement. tidb#38671
• 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

[jackysp]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-19 18:26:41.294637151 +0000 UTC m=+40966.695887600: ☑️ agreed by jackysp.
• 2023-06-20 03:49:09.235125085 +0000 UTC m=+74714.636375534: ☑️ agreed by ran-huang.

[ran-huang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ran-huang

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #14285.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #14286.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14287.