docs: add memory unit example for tidb_server_memory_limit

[doc-claw-bot]

What is changed, added or deleted? (Required)

• Add a 90GiB example to the tidb_server_memory_limit description in system-variables.md.
• Clarify the documented unit-format example without changing the supported formats.

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• v6.5 (TiDB 6.5 versions)

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

• This PR is translated from:
• Other reference link(s): docs: add memory unit example for tidb_server_memory_limit docs#23587

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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

Summary by CodeRabbit

• 文档
  • 更新 tidb_server_memory_limit 和 tidb_server_memory_limit_sess_min_size 的内存大小格式说明。
  • 新增支持 KB、MB、GB 和 TB 单位。
  • 补充 90GiB 配置示例。

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📥 Commits

Reviewing files that changed from the base of the PR and between 495c5fa and 95d484d.

📒 Files selected for processing (1)

• system-variables.md

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📝 Walkthrough

Walkthrough

更新两个内存相关系统变量的文档说明。新增 KB|MB|GB|TB 单位格式，并补充 90GiB 示例。

Changes

内存值格式说明

Layer / File(s)	Summary
更新内存变量格式说明 system-variables.md	为 tidb_server_memory_limit 和 tidb_server_memory_limit_sess_min_size 增加 `KB

Estimated code review effort: 1 (简单) | ~2 分钟

Merge Risk: ⚪ Minimal · up to 95d48

This PR only clarifies the documented memory-unit example for tidb_server_memory_limit without changing product behavior or supported formats; no actionable merge-blocking risk remains.

Suggested reviewers: qiancai, lilin90

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题说明了为 tidb_server_memory_limit 添加内存单位示例，与主要变更相关。标题未提及新增 KB、MB、GB、TB 单位，但不影响其通过。
Description check	✅ Passed	描述包含变更内容、适用版本、相关 PR、AI 参与情况和变更类型检查项。内容与 PR 目标基本一致，但“without changing the supported formats”与实际新增 KB、MB、GB、TB 单位的变更摘要不一致。
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
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.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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