docs: add statement summary system variables

[nolouch]

What is changed

This PR is translated from: pingcap/docs-cn#21694

Add English documentation for two statement summary system variables, corresponding to pingcap/docs-cn#21694:

• tidb_stmt_summary_group_by_user
• tidb_stmt_summary_persist_evicted

Updated files:

• statement-summary-tables.md
• system-variables.md
• system-variable-reference.md

Check List

Tests:

• ./scripts/markdownlint statement-summary-tables.md system-variable-reference.md system-variables.md
• git diff --check -- statement-summary-tables.md system-variable-reference.md system-variables.md

Summary by CodeRabbit

• New Features

  • Added tidb_stmt_summary_group_by_user to optionally separate statement summaries by executing user.
  • Added tidb_stmt_summary_persist_evicted to optionally preserve LRU-evicted statement records with an evicted marker.
  • Documented defaults, memory and logging behavior, data clearing, and version-specific guidance.

• Documentation

  • Added system-variable references and links for both statement summary settings.
  • Clarified that evicted records are logged as JSON and excluded from history query results.

[gemini-code-assist]

Code Review

This pull request adds documentation for two new system variables introduced in TiDB v8.5.7: tidb_stmt_summary_group_by_user and tidb_stmt_summary_persist_evicted. The changes span across statement-summary-tables.md, system-variable-reference.md, and system-variables.md. The review comments suggest rewriting several sentences to use the active voice, reducing wordiness, and using "who" instead of "that" when referring to users, in alignment with the repository's documentation style guide.

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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: e2f8fcdd-340d-4849-b7a3-6b1d0f5e519e

📥 Commits

Reviewing files that changed from the base of the PR and between c86061a and 7c8957b.

📒 Files selected for processing (3)

• statement-summary-tables.md
• system-variable-reference.md
• 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 adds two GLOBAL statement summary variables. It describes user-based aggregation, LRU-evicted record persistence, defaults, memory effects, log output, and history query behavior.

Changes

Statement summary configuration

Layer / File(s)	Summary
System variable references and definitions system-variables.md, system-variable-reference.md	Adds tidb_stmt_summary_group_by_user and tidb_stmt_summary_persist_evicted as GLOBAL Boolean variables with defaults, effects, and related links.
Statement summary behavior statement-summary-tables.md	Documents user aggregation, SAMPLE_USER, in-memory clearing, persisted eviction logs, increased log volume, and exclusion from history queries.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 7c895

The PR only adds documentation for two system variables. No actionable merge-blocking risk remains; the normal link-anchor check should confirm that the new references resolve correctly.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the changes and lists validation, but it omits the required affected TiDB version section and other template fields.	Add the required TiDB version selection and complete the applicable template sections, including related links and change-impact checkboxes.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the addition of statement summary system variables to the documentation.
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. (3 skipped: 3 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 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[qiancai]

Suggested change

	> **Note:**
	>
	> This variable is supported only for TiDB Cloud Essential and Premium instances whose kernel version is TiDB-X-CLOUD.202603.1 or later.