Clarify Local-backend in tidb-lightning-backends.md (#5278)

[ti-srebot]

cherry-pick #5278 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In docs repo:
git pr https://github.com/pingcap/docs/pull/5279

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

git push [email protected]:ti-srebot/docs.git pr/5279:release-4.0-868f416aebc2

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[ti-srebot]

@yikeke please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs/invitations

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• yikeke

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[yikeke]

/merge

[ti-chi-bot]

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

Commit hash: 534b568