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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Summary by CodeRabbit

• Documentation
  • Clarified that tidb_server_memory_limit accepts memory values with explicit units.
  • Added a 90GiB configuration example without a space between the value and unit.

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• system-variables.md

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📝 Walkthrough

Walkthrough

The tidb_server_memory_limit documentation now includes a 90GiB example and states that no space is allowed between the value and unit.

Changes

Memory limit documentation

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

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

Merge Risk: ⚪ Minimal · up to 29383

This localized documentation change only clarifies a memory-unit example and introduces no actionable merge-blocking risk after normal checks and review.

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

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description explains the documentation change, affected version, reference PR, and change classifications. The optional first-time contributor checklist and AI agent involvement section are omitte…
Title check	✅ Passed	The title clearly and concisely describes the primary documentation change: adding a memory-unit example for tidb_server_memory_limit.
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: Description check

Explanation

The description explains the documentation change, affected version, reference PR, and change classifications. The optional first-time contributor checklist and AI agent involvement section are omitted, but the required content is mostly complete.

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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[qiancai]

/approve

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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Needs approval from an approver in each of these files:

• OWNERS [qiancai]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #23657.