flushSync: Translate untranslated parts

[smikitky]

#452 (comment)

上記のコメントに対応

[Copilot]

Pull Request Overview

This PR translates untranslated English text in the flushSync documentation to Japanese, addressing issue #452. The changes complete the localization of the "Caveats" section.

• Translates four English bullet points about flushSync behavior and performance considerations into Japanese

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[github-actions]

Size changes

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
legacy-ja-reactjs-org	Ignored			Oct 21, 2025 10:58am

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[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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