translate two PRs from pingcap/docs-cn

[qiancai]

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)

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.

• master (the latest development version)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: add config unit for tidb_adaptive_closest_read_threshold docs-cn#13697 and system-variables.md: add version tag for tidb_enable_ddl docs-cn#13509
• 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-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hfxsd
• tangenta

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After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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[qiancai]

/merge

[ti-chi-bot]

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

Commit hash: d27cc29

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.6: #13423.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.0: #13424.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #13425.