docs: add memory unit example for tidb_server_memory_limit (#21893)

[ti-chi-bot]

This is an automated cherry-pick of #21893

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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• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
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• v7.1 (TiDB 7.1 versions)
• 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 的格式说明。
  • 支持使用 KB、MB、GB、TB 单位配置内存值。
  • 保留原有二进制单位格式支持。

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📝 Walkthrough

Walkthrough

更新 tidb_server_memory_limit 和 tidb_server_memory_limit_sess_min_size 的内存值格式说明，新增 KB、MB、GB、TB 单位，并保留原有二进制单位格式。

Changes

系统变量内存单位格式更新

Layer / File(s)	Summary
更新系统变量格式说明 system-variables.md	为两个系统变量新增 KB、MB、GB、TB 单位说明，并保留 KiB、MiB、GiB、TiB 格式。

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

Merge Risk: ⚪ Minimal · up to 22476

This change only clarifies the documented memory-limit unit example and does not alter product behavior; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: qiancai

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	描述包含变更内容、版本选择、相关链接和 AI 参与情况，但版本选择与 PR 目标不一致。该 PR 目标是自动回移到 release-8.5 分支，而描述仅勾选了 master。相关 PR 编号也存在不一致。	勾选 v8.5，并根据实际来源补充或更正相关 PR 链接。若同时应用于 master，请一并勾选 master。

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	标题准确概括了主要文档变更，即为 tidb_server_memory_limit 添加内存单位示例。标题简洁、明确，并与 PR 目标相关。
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.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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• system-variables.md

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