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: docs: add memory unit example for tidb_server_memory_limit docs-cn#21893
• Other reference link(s): N/A

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

• Documentation
  • Clarified that tidb_server_memory_limit accepts memory values with explicit units.
  • Added a 90GiB configuration example.

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ℹ️ Recent review info

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Run ID: 780817a7-f74b-458d-9914-b818443f5871

📥 Commits

Reviewing files that changed from the base of the PR and between e5b0fe8 and a516f4e.

📒 Files selected for processing (1)

• system-variables.md

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📝 Walkthrough

Walkthrough

The tidb_server_memory_limit documentation now includes explicit memory units with a 90GiB example.

Changes

Memory limit documentation

Layer / File(s)	Summary
Document memory-size syntax system-variables.md	Adds a 90GiB example while retaining the existing byte-range and unit rules.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to a516f

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

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the documentation change to add a memory-unit example.
Description check	✅ Passed	The description covers the required change, affected version, and reference link; optional checklist sections are omitted.
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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