docs: add tidb_enable_connection_event_log system variable

[gengliqi]

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Add tidb_enable_connection_event_log system variable

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Summary by CodeRabbit

• 新功能

  • 新增系统变量 tidb_enable_connection_event_log，用于控制是否将客户端成功登录和退出事件写入 TiDB 日志。
  • 该变量支持全局作用域和集群持久化，默认值为 OFF，不支持通过 SET_VAR Hint 设置。

• 文档

  • 补充高频建立或关闭连接可能产生大量日志的说明。

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: ASSERTIVE

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Run ID: 9288336c-f292-44ca-9cb0-323a29580174

📥 Commits

Reviewing files that changed from the base of the PR and between 5a1d9cb and 5aae76b.

📒 Files selected for processing (1)

• system-variables.md

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📝 Walkthrough

Walkthrough

新增系统变量 tidb_enable_connection_event_log 的文档说明。该变量控制客户端成功登录和退出事件是否写入 TiDB 日志。

Changes

连接事件日志变量

Layer / File(s)	Summary
变量定义与日志行为说明 system-variables.md	新增 tidb_enable_connection_event_log 的 GLOBAL 作用域、集群持久化方式、默认值 OFF 和 SET_VAR Hint 限制。文档说明启用后会记录客户端成功登录和退出事件。

Estimated code review effort: 1（简单） | ~5 分钟

Mergeability Score: ⚪ Minimal · up to 5aae7

This documentation-only change adds a system variable entry and has no actionable merge-blocking risk remaining after normal checks and review.

Suggested reviewers: qiancai, lilin90, ti-chi-bot

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了新增系统变量文档的主要变更，内容清晰且简洁。
Description check	✅ Passed	描述包含变更内容、适用版本和必需检查项，整体信息基本完整。
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.

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