Update garbage collection docs #4552 (#5856)

[ti-srebot]

cherry-pick #5856 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/5867

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

git push [email protected]:ti-srebot/docs-cn.git pr/5867:release-5.0-c07d24883c30

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First-time contributors' checklist

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What is changed, added or deleted? (Required)

This updates the docs for pingcap/tidb#21988 (now merged).

21988 still allows the previous methods for updating, but because these changes are not validated, they should not be recommended. Incorrect configuration can lead to strange behaviors.

Partially address #4549 (for backup page mentioned, there are still other pages that need readability improvements)

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)
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• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from: Update garbage collection docs docs#4552
• 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

[ti-srebot]

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

[ran-huang]

/lgtm

[ti-chi-bot]

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[ran-huang]

/merge

[ti-chi-bot]

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

Commit hash: c023624