Add restriction information for index merge to avoid misuse

[ichn-hu]

First-time contributors' checklist

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

What is changed, added or deleted? (Required)

add the restriction to avoid misuse.

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

Tips for choosing the affected version(s):

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).

• [x] master (the latest development version)
• [x] v5.2 (TiDB 5.2 versions)
• [x] v5.1 (TiDB 5.1 versions)
• [x] v5.0 (TiDB 5.0 versions)
• [x] v4.0 (TiDB 4.0 versions)
• [ ] v3.1 (TiDB 3.1 versions)
• [ ] v3.0 (TiDB 3.0 versions)
• [ ] v2.1 (TiDB 2.1 versions)

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

• This PR is translated from:
• 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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[TomShawn]

@ichn-hu Could you please involve a technical reviewer? Thanks~

[ichn-hu]

@qw4990 PTAL

[ti-chi-bot]

@ichn-hu: PR needs rebase.

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[TomShawn]

/label needs-cherry-pick-release-6.0

[qiancai]

Removed the needs-cherry-pick-release-6.4 label because the v6.4 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v6.4 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-6.3 label because the v6.3 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v6.3 and will no longer receive new updates.

[hfxsd]

这些内容已经在最新的文档里有了。另外现在已经支持事务里面使用 index merge。