i18n(ja): unify remaining Read Committed isolation level notation

[yahonda]

What is changed, added or deleted? (Required)

Follow-up to #23639 (which unified the Repeatable Read isolation level term and part of the Read Committed backlog). This PR converts the remaining Japanese-only / inconsistently-cased renderings of the SQL standard "Read Committed" isolation level to English, verified per-site against the corresponding English text and mirroring each site's own EN casing (e.g. Title Case "Read Committed" vs lowercase-hyphenated "read-committed", depending on how EN itself renders it at that specific site).

Also fixed one same-paragraph self-contradiction found in passing in releases/release-6.3.0.md: a bullet heading already correctly said "Read-Committed" while the very next line's body prose still said the katakana form.

Deliberately excludes transaction-isolation-levels.md and tidb-performance-tuning-config.md — both already fixed by the still-open #23639 on an unmerged branch; touching them here would conflict.

5 files changed, 6 sites converted.

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

• i18n-ja-release-8.5 (TiDB Japanese documentation for TiDB 8.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): follow-up to i18n(ja): keep SQL transaction isolation level names in English #23639

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

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
  • Standardized Read Committed and read-committed terminology across transaction isolation, system variable, and release documentation.
  • Clarified descriptions of Read Committed behavior, TSO-related features, and TiCDC’s default isolation level.
  • Improved wording and link labels while preserving existing documentation links, version availability, and documented configuration scopes.

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ℹ️ Recent review info

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Run ID: 17e1a0d7-4308-4db4-84e4-ff7532645b43

📥 Commits

Reviewing files that changed from the base of the PR and between a349332 and f360274.

📒 Files selected for processing (5)

• basic-features.md
• releases/release-2.0.6.md
• releases/release-6.0.0-dmr.md
• releases/release-6.3.0.md
• system-variables.md

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📝 Walkthrough

Walkthrough

The PR standardizes Read Committed terminology across transaction isolation documentation, system-variable descriptions, and release notes. It also refines wording for TSO-fetch behavior and corrects one Japanese phrase without changing links or configuration semantics.

Changes

Read Committed documentation terminology

Layer / File(s)	Summary
Reference terminology updates basic-features.md, system-variables.md	Transaction isolation labels and system-variable descriptions use consistent Read Committed terminology. Existing links and configuration semantics remain unchanged.

Release note terminology updates

Layer / File(s)	Summary
Release note terminology updates releases/release-2.0.6.md, releases/release-6.0.0-dmr.md, releases/release-6.3.0.md	Release notes use consistent Read Committed wording and refine TSO-fetch descriptions. Documented behavior and issue references remain unchanged.

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

Merge Risk: ⚪ Minimal · up to f3602

This PR standardizes Read Committed terminology in Japanese documentation without changing product behavior, so no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: qiancai, invalid-email-address

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the Japanese internationalization change and the unification of remaining Read Committed isolation-level terminology.
Description check	✅ Passed	The description explains the changes, affected documentation version, related PR, AI involvement, and applicable checklist items. It is complete and directly related to the pull request.
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: 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. (5 skipped: 5 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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[LGTM Timeline notifier]

Timeline:

• 2026-08-28 08:22:35.045341419 +0000 UTC m=+835590.216435565: ☑️ agreed by qiancai.