Update clustered-indexes.md (#5771)

[ti-srebot]

cherry-pick #5771 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In docs-cn repo:
git pr https://github.com/pingcap/docs-cn/pull/5805

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs-cn.git pr/5805:release-5.0-6876ad22ff82

---

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)

Update the user documentation of clustered index feature introduced in v5.0-rc & v5.0.

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

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• 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: NA
• Other reference link(s):
  • clustered index project: https://github.com/pingcap/tidb/projects/45

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-srebot]

@tangenta you're already a collaborator in bot's repo.

[TomShawn]

/status DNM

[TomShawn]

/remove-status DNM
/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: 52b140a