tidb: correct some variables descriptions

[0xPoe]

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ref pingcap/tidb#64023

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[gemini-code-assist]

Summary of Changes

Hello @0xPoe, 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 clarifies and corrects the documentation surrounding concurrency control variables in TiDB. It addresses the deprecation of tidb_index_serial_scan_concurrency by updating its description to reflect its read-only status and guiding users to the appropriate replacement variables, tidb_executor_concurrency for sequential index scans and tidb_analyze_distsql_scan_concurrency for index statistics collection. The changes ensure that users are directed to the correct variables for optimizing performance and troubleshooting.

Highlights

• Variable Deprecation: The tidb_index_serial_scan_concurrency system variable has been deprecated and is now read-only, serving as an alias for tidb_executor_concurrency.
• Concurrency Control Updates: Sequential index scans are now controlled by tidb_executor_concurrency, while index statistics collection (e.g., during ANALYZE TABLE) uses tidb_analyze_distsql_scan_concurrency.
• Documentation Alignment: Various documentation files, including best practices, system variable descriptions, TPC-C benchmarking guides, TiDB Lightning configuration, and OOM troubleshooting, have been updated to reflect these changes and guide users to the correct variables.

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[gemini-code-assist]

Code Review

This pull request correctly updates the documentation to reflect the deprecation of tidb_index_serial_scan_concurrency and points to the new variables to use. The changes are consistent across multiple files. I've provided a couple of suggestions to improve clarity and conciseness by rephrasing some descriptions to be more direct and to avoid repetition, in line with the repository's style guide.

[gemini-code-assist]

This sentence is a bit long and could be more direct. To improve clarity and follow the style guide's principle of using the second person, consider rephrasing.¹

Suggested change

	[`tidb_index_serial_scan_concurrency`](/system-variables.md#tidb_index_serial_scan_concurrency) is deprecated and read-only. Index statistics collection now follows [`tidb_analyze_distsql_scan_concurrency`](/system-variables.md#tidb_analyze_distsql_scan_concurrency), so adjust that variable to change the number of Regions read for index statistics.
	[`tidb_index_serial_scan_concurrency`](/system-variables.md#tidb_index_serial_scan_concurrency) is deprecated and read-only. To control the number of Regions read for index statistics collection, you can adjust [`tidb_analyze_distsql_scan_concurrency`](/system-variables.md#tidb_analyze_distsql_scan_concurrency) instead.

Style Guide References

Footnotes

1. The style guide recommends writing in the second person ('you') and avoiding unnecessary words for better clarity. ↩