dm: add LOCK TABLES privilege note for managed MySQL sources

[alastori]

What is changed, added or deleted?

Added conditional LOCK TABLES privilege documentation for managed MySQL sources (Amazon RDS, Aurora, Google Cloud SQL) across three DM docs pages.

Background: DM defaults to consistency=auto. On managed MySQL where FLUSH TABLES WITH READ LOCK is restricted by the cloud provider, DM falls back to LOCK TABLES. This privilege is not needed on self-managed MySQL instances. Confirmed with @GMHDBJD (Minghao Guo): the FTWRL → LOCK TABLES fallback in auto mode is by design.

Changes:

• dm/dm-precheck.md: Clarified that LOCK TABLES is needed for auto fallback on managed MySQL, not just flush/lock
• dm/dm-worker-intro.md: Added LOCK TABLES to privilege table with managed-MySQL scope note; added conditional GRANT example
• dm/quick-start-with-dm.md: Added note pointing to dm-worker-intro for managed MySQL sources (×2 instances)

Evidence: Lab-06: LOCK TABLES privilege testing (9 scenarios, vanilla MySQL vs RDS)

Related:

• Cloud DM docs: cloud/dm: add LOCK TABLES to source privilege prerequisites #22598
• Pre-check improvement: https://tidb.atlassian.net/browse/DM-12687

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

• master (dev)
• v8.5 (LTS)

cc @GMHDBJD @qiancai @OliverS929

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the documentation for TiDB Data Migration (DM) to provide clearer guidance on LOCK TABLES privilege requirements for users migrating data from managed MySQL services such as Amazon RDS or Aurora. It addresses the scenario where DM's consistency=auto mode falls back to LOCK TABLES when FLUSH TABLES WITH READ LOCK is restricted by cloud providers, ensuring users correctly configure necessary permissions for successful data migration.

Highlights

• dm/dm-precheck.md update: Clarified the requirement for LOCK TABLES privilege when consistency=auto is used with managed MySQL services (like Amazon RDS or Aurora) due to FLUSH TABLES WITH READ LOCK restrictions.
• dm/dm-worker-intro.md update: Incorporated a note and a conditional GRANT example for the LOCK TABLES privilege, specifically for users migrating from managed MySQL services.
• dm/quick-start-with-dm.md update: Added two notes directing users to the dm-worker-intro.md page for details on LOCK TABLES privileges when using managed MySQL sources.

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Code Review

The pull request enhances the documentation for TiDB Data Migration (DM) by providing clearer guidance on the "LOCK TABLES" privilege, especially for managed MySQL services like Amazon RDS and Aurora. This addresses a specific scenario where DM's consistency=auto mode falls back to LOCK TABLES when FLUSH TABLES WITH READ LOCK is restricted. The changes improve the completeness and accuracy of the privilege requirements across several key documentation pages, making the information more precise and helpful for users.