docs: clarify Syncpoint changefeed configuration scope

[doc-claw-bot]

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)

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 (in Chinese).

• 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:
• Other reference link(s): docs: clarify Syncpoint changefeed configuration scope docs#23363

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

[coderabbitai]

📝 Walkthrough

Walkthrough

本次更新统一 Syncpoint 的配置归属表述，并明确备用集群设置已复制完成一致性快照点的方式与周期。

Changes

Syncpoint 文档更新

Layer / File(s)	Summary
Syncpoint 配置与流程说明 dr-secondary-cluster.md, ticdc/ticdc-upstream-downstream-check.md	将 enable-sync-point 明确为 changefeed 配置项，并补充 Changefeed 按 sync-point-interval 设置备用集群一致性快照点的说明。

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	标题准确概括了将 Syncpoint 配置范围明确为 changefeed 配置的主要改动。
Description check	✅ Passed	描述包含了变更内容、影响版本、相关链接和 AI 参与信息，结构基本完整。

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-07-28 02:12:14.432261069 +0000 UTC m=+1889320.468356135: ☑️ agreed by qiancai.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21839.
But this PR has conflicts, please resolve them!