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 a setting to group statement summaries by executing user.
  • Added an option to preserve LRU-evicted statement summary records with an evicted marker.
  • Documented defaults and availability across TiDB and TiDB Cloud editions, including Support requirements for modifying read-only settings.

• Documentation

  • Added system-variable references and links for both statement summary settings.
  • Clarified the location and presentation of the eviction-related documentation note.

[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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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 88bef9a9-dd18-4efc-a163-699c9594bf59

📥 Commits

Reviewing files that changed from the base of the PR and between eb39905 and 8f92301.

📒 Files selected for processing (1)

• statement-summary-tables.md

🚧 Files skipped from review as they are similar to previous changes (1)

• statement-summary-tables.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, history query behavior, and TiDB Cloud restrictions.

Changes

Statement summary configuration

Layer / File(s)	Summary
System variable references and definitions system-variables.md, system-variable-reference.md	Adds both GLOBAL Boolean variables with defaults, effects, TiDB Cloud restrictions, 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 8f923

This PR adds documentation for two statement summary system variables without changing product behavior or runtime configuration; no actionable merge-blocking risk remains after normal checks and review.

Poem

A rabbit checks each summary note
Two new settings now take root
Users shape the records
Evictions mark the logs
Clear guidance keeps the tables clear

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding statement summary system variables to the documentation.
Description check	✅ Passed	The description explains the changes, identifies the affected files, provides the translated source PR, and records completed checks. It omits the required affected-version checklist and other templat…
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 changes, identifies the affected files, provides the translated source PR, and records completed checks. It omits the required affected-version checklist and other template checklists, but it is otherwise sufficiently complete and on topic.

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 💡 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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