system variables: document tidb_opt_index_join_build_v2 compat

[qiancai]

What is changed, added or deleted? (Required)

Document the compatibility behavior of tidb_opt_index_join_build_v2.

• Add the system variable entry in system-variables.md.
• Clarify that TiDB always uses index join build v2 now.
• Clarify that setting this variable to OFF returns an error and cannot switch back to v1.
• Clarify that historical values left in mysql.global_variables after upgrade are still treated as ON.
• Add the corresponding entry to system-variable-reference.md.

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)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: system variables: document tidb_opt_index_join_build_v2 compat docs-cn#21434
• Other reference link(s):
  • planner, sessionctx: remove index join build v1 tidb#66871

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

[gemini-code-assist]

Code Review

This pull request adds a placeholder file named temp.md. The reviewer noted that this file should be removed as it does not contain the documentation for tidb_opt_index_join_build_v2 mentioned in the pull request description, and suggested placing the documentation in the correct system variable files instead.

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temp.md (1)

This file appears to be a placeholder and does not contain the documentation for tidb_opt_index_join_build_v2 as described in the pull request title and description. Remove this file and include the documentation in the appropriate files (system-variables.md and system-variable-reference.md).

References

1. The documentation should be complete and accurate, covering all aspects described in the pull request summary. ^(link)

[github-actions]

Auto-sync completed successfully

Source PR: pingcap/docs-cn#21434
Target PR: #22739

English documentation has been updated based on Chinese documentation changes.

[ti-chi-bot]

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