docs: add memory unit example for tidb_server_memory_limit (#23587)

[ti-chi-bot]

This is an automated cherry-pick of #23587

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.

Which TiDB version(s) do your changes apply to? (Required)

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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
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Summary by CodeRabbit

• Documentation
  • Updated memory-size configuration guidance to support decimal units (KB, MB, GB, TB) alongside binary units.
  • Added a 90GiB usage example.

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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: f12acf43-4b90-49ce-9a26-014b58476cce

📥 Commits

Reviewing files that changed from the base of the PR and between 965cdb9 and 0769740.

📒 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

The documentation for two TiDB system variables now includes decimal memory units (KB|MB|GB|TB) alongside binary units and adds a 90GiB example.

Changes

Memory-size documentation

Layer / File(s)	Summary
Document supported memory-size formats system-variables.md	The descriptions for tidb_server_memory_limit and tidb_server_memory_limit_sess_min_size now list decimal units and include a 90GiB example.

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

Merge Risk: ⚪ Minimal · up to 07697

This change only clarifies the documented memory-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	The description includes the required change summary, version section, reference link, and change-type checklist. However, it selects master while the PR objective identifies this as an automated cher…	Confirm the target version. Select v8.5 instead of master if this PR targets the release-8.5 branch, or update the objective to explain why master is correct.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation change and the specific memory-unit example added.
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 includes the required change summary, version section, reference link, and change-type checklist. However, it selects master while the PR objective identifies this as an automated cherry-pick for the release-8.5 branch.

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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