restore: update the definition of the parameter --load-stats and the usage of pitr id map (#21078)

[ti-chi-bot]

This is an automated cherry-pick of #21078

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This PR brings some updates added in #20492, which is also cherry picked into v8.5, as confirmed by @Leavrth.

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• This PR is translated from: restore: update the definition of the parameter --load-stats and the usage of pitr id map docs-cn#20346
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[ti-chi-bot]

@Leavrth This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, 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 updates the documentation for TiDB's Backup & Restore (BR) tool, enhancing clarity around Point-in-Time Recovery (PITR) ID map management and introducing new features for checkpoint data storage and system table restoration. The changes provide users with more detailed guidance on configuring and understanding BR's behavior, particularly for newer versions, and introduce a more efficient method for restoring system tables.

Highlights

• PITR ID Map Handling: The documentation for mysql.tidb_pitr_id_map has been updated to clarify its persistence, noting that from v9.0.0, if the table doesn't exist and --checkpoint-storage is not specified, the pitr_id_map data will be written to the log backup directory.
• External Checkpoint Storage: New documentation has been added detailing the --checkpoint-storage parameter, which allows specifying an external storage location for checkpoint data, along with a description of the directory structure used for these checkpoints.
• Physical System Table Restoration: The --fast-load-sys-tables parameter has been introduced (from v9.0.0), enabling physical restoration of system tables using RENAME TABLE DDL, which atomically swaps temporary system tables with those in the mysql database.
• Load Statistics Parameter Update: The behavior of the --load-stats parameter has been clarified for v9.0.0; when set to false, BR no longer writes statistics to mysql.stats_meta, requiring manual ANALYZE TABLE post-restore.

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[gemini-code-assist]

Code Review

This pull request updates documentation related to restore operations, specifically for checkpoint restore and snapshot restore. It introduces new features available from v9.0.0, such as storing checkpoint data in external storage and using --fast-load-sys-tables for physical restore of system tables.

My review focuses on improving the clarity and structure of the documentation. I've identified a critical issue where merge conflict markers were left in br/br-checkpoint-restore.md, which will break the document rendering. I've also suggested a significant restructuring of that document to avoid confusion caused by duplicated section headings and to better present the different methods for storing checkpoint data. Additionally, I've provided suggestions to correct inaccuracies (like 'target cluster' instead of 'external storage'), improve sentence structure for better readability, and ensure consistent formatting.

[ti-chi-bot]

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