docs: clarify Syncpoint changefeed configuration scope (#23363)

[ti-chi-bot]

This is an automated cherry-pick of #23363

What is changed, added or deleted? (Required)

Clarify that enable-sync-point is a changefeed configuration item rather than a TiCDC server configuration item.

Update the related Syncpoint wording in these docs so readers know the setting belongs to the changefeed configuration file:

• ticdc/ticdc-upstream-downstream-check.md
• dr-secondary-cluster.md

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

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• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from: docs: clarify Syncpoint changefeed configuration scope docs-cn#21838
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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

Summary by CodeRabbit

• Documentation
  • Clarified that Syncpoint must be enabled in the changefeed configuration file.
  • Updated guidance to identify enable-sync-point as a changefeed configuration item.
  • Refined instructions for aligning consistent snapshot points between clusters.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between a0ff8ef and 6ae9a52.

📒 Files selected for processing (2)

• dr-secondary-cluster.md
• ticdc/ticdc-upstream-downstream-check.md

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📝 Walkthrough

Walkthrough

Updated two Syncpoint documentation pages to specify that enable-sync-point must be configured in the changefeed configuration.

Changes

Syncpoint documentation

Layer / File(s)	Summary
Clarify changefeed configuration dr-secondary-cluster.md, ticdc/ticdc-upstream-downstream-check.md	Specifies that Syncpoint and enable-sync-point must be set in the changefeed configuration while preserving the existing tidb_external_ts behavior description.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Possibly related PRs

• pingcap/docs#23363: Updates the same documentation files with closely related clarification of changefeed-level Syncpoint configuration.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the doc update about clarifying Syncpoint configuration scope.
Description check	✅ Passed	The description covers the change, affected version, source PR, and AI involvement, meeting the required template.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Create PR with unit tests

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