docs: clarify TiCDC index DDL ordering (#23336)

[ti-chi-bot]

This is an automated cherry-pick of #23336

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 that ADD INDEX and CREATE INDEX statements are executed asynchronously downstream and return before completion.
  • Updated execution-order guidance to note that most DDL statements are serialized, with TiDB-specific exceptions documented.
  • Added a reference to detailed asynchronous DDL behavior.

[coderabbitai]

📝 Walkthrough

Walkthrough

The TiCDC documentation now clarifies that most DDL statements usually execute serially, while ADD INDEX and CREATE INDEX run asynchronously on downstream TiDB clusters.

Changes

TiCDC index DDL documentation

Layer / File(s)	Summary
Execution order and asynchronous index DDLs ticdc/ticdc-faq.md, ticdc/ticdc-ddl.md	The documentation clarifies usual DDL ordering and states that TiCDC returns before downstream TiDB completes ADD INDEX or CREATE INDEX.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Possibly related PRs

• pingcap/docs#23336: Makes the same TiCDC documentation updates for asynchronous index DDL handling and execution ordering.

Suggested labels: type/enhancement, area/ticdc, translation/from-docs-cn

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: clarifying TiCDC index DDL ordering.
Description check	✅ Passed	The description explains the changes, affected version, references, AI involvement, and applicable checklist items.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 1

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ℹ️ Review info

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Run ID: 9c482e56-51dc-4a7f-9313-e535626ab236

📥 Commits

Reviewing files that changed from the base of the PR and between 9c03d3a and 11a03f1.

📒 Files selected for processing (2)

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

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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