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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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
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• 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 的配置示例，新增 90GiB 格式。
  • 明确支持带单位的内存值。

[ti-chi-bot]

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 6aff6a07-ae56-44f1-8ec6-64a223526b18

📥 Commits

Reviewing files that changed from the base of the PR and between 8fcf0ed and 495c5fa.

📒 Files selected for processing (1)

• system-variables.md

Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review.

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📝 Walkthrough

Walkthrough

更新 tidb_server_memory_limit 的文档示例，新增 90GiB，说明该变量支持带单位的内存值。原有取值范围和 0 的含义保持不变。

Changes

内存值格式示例

Layer / File(s)	Summary
更新内存值示例 system-variables.md	为 tidb_server_memory_limit 增加 90GiB 示例，并保留原有取值说明。

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

Merge Risk: ⚪ Minimal · up to 495c5

This PR only clarifies a documented memory-unit example and does not change product behavior; 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	标题准确概括了为文档增加 90GiB 内存单位示例的主要变更。
Description check	✅ Passed	描述包含变更内容、适用版本、关联链接及 AI 参与情况，并完整填写了模板要求的版本选择。
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 files. (1 skipped: 1 unsupported.)
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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