docs: clarify TiCDC index DDL ordering

[doc-claw-bot]

First-time contributors' checklist

• I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

• Clarify in the TiCDC FAQ that the general DML/DDL execution order has an exception for ADD INDEX and CREATE INDEX when the downstream is TiDB.
• Update the TiCDC DDL page to explicitly call out these two DDLs as exceptions so the FAQ and DDL pages stay consistent.

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

• 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 TiCDC index DDL ordering docs-cn#21829
  • https://docs.pingcap.com/tidb/dev/ticdc-faq/
  • https://docs.pingcap.com/tidb/dev/ticdc-ddl/

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 TiCDC’s execution order for DDL vs. DML replication as a three-step flow anchored to the DDL commit timestamp.
  • Documented that replication pauses for tables affected by DDL until the DDL commit completes.
  • Added TiDB-specific note that ADD INDEX and CREATE INDEX run asynchronously, allowing subsequent DML to proceed without waiting for completion.

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📥 Commits

Reviewing files that changed from the base of the PR and between 348fac6 and a58a942.

📒 Files selected for processing (2)

• ticdc/ticdc-ddl.md
• ticdc/ticdc-faq.md

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📝 Walkthrough

Walkthrough

TiCDC documentation now explains DML and DDL replication ordering, including commit timestamp handling and TiDB-specific asynchronous execution of ADD INDEX and CREATE INDEX.

Changes

TiCDC ordering documentation

Layer / File(s)	Summary
Ordering and asynchronous index DDL ticdc/ticdc-faq.md, ticdc/ticdc-ddl.md	The FAQ describes blocking affected-table DML through the DDL commitTS and resuming afterward; the DDL guide clarifies that TiDB index DDL statements execute asynchronously as exceptions to ordered replication.

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main change by highlighting TiCDC index DDL ordering clarification.
Description check	✅ Passed	The description follows the template with the required change summary, version, links, AI involvement, and checklist sections.
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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[qiancai]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #23417.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #23418.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #23419.