docs: fix tidb_server_memory_limit unit description (release-8.5)

[vjymisal0]

What is changed, added or deleted? (Required)

• Fix the tidb_server_memory_limit unit description in system-variables.md on the release-8.5 branch, matching the wording already fixed on master (docs: add memory unit example for tidb_server_memory_limit #23587) and backported to release-7.5 (docs: fix tidb_server_memory_limit unit description #23588).
• Clarify that both KiB|MiB|GiB|TiB and KB|MB|GB|TB unit formats are supported, and add a 90GiB example.

This gap exists on release-7.6 through release-8.4 as well; happy to open matching PRs for those branches if useful, or this could go through the needs-cherry-pick-release-* bot flow instead.

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

• release-8.5

Summary by CodeRabbit

• Documentation
  • Updated memory limit variable documentation to support both binary and decimal size units.
  • Added an example clarifying that no space is allowed between the numeric value and unit.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 965cdb9 and a6bbd53.

📒 Files selected for processing (1)

• system-variables.md

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📝 Walkthrough

Walkthrough

The documentation updates two TiDB system variable descriptions. Both descriptions list binary and decimal memory units and add a 90GiB example that shows no space is allowed between the number and unit.

Changes

Memory unit documentation

Layer / File(s)	Summary
Update memory variable descriptions system-variables.md	The tidb_server_memory_limit and tidb_server_memory_limit_sess_min_size descriptions now list KiB|MiB|GiB|TiB and KB|MB|GB|TB formats. Each description includes the 90GiB no-space example.

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

Merge Risk: ⚪ Minimal · up to a6bbd

This localized documentation update clarifies supported memory-limit units and adds an example; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: ti-chi-bot, qiancai

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the documentation fix and the affected release branch.
Description check	✅ Passed	The description explains the documentation change, its purpose, affected version, and related backport context. The required sections are sufficiently complete; 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…
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, its purpose, affected version, and related backport context. The required sections are sufficiently complete; optional checklist sections are omitted.

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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