update the description related to clustered indexes (#5834)

[ti-srebot]

cherry-pick #5834 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/5858

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

git push git@github.com:ti-srebot/docs-cn.git pr/5858:release-5.0-9c7ddf06bf83

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

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• Other reference link(s): Update clustered-indexes.md #5771 *: remove alter-primary-key configuration #5774

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[ti-srebot]

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

[TomShawn]

/lgtm

[ti-chi-bot]

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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: 1d1778d