docs(experiments): clarify optimization target defaults

[tonoizer]

Description

Correct the experiments.optimization.target documentation to match the current Webpack and Rspack plugin behavior:

• The default is unset; ENV_TARGET is not inferred from the bundler's target option.
• Users should set target: 'web' explicitly for browser builds so Node.js-specific loader code can be removed at compile time.
• Users should set target: 'node' explicitly for Node.js builds, including SSR scenarios.

Search findings:

• #4452 describes ENV_TARGET as a compile-time value supplied by the build configuration and preserves a runtime fallback when it is absent.
• #3704 initially added target inference, but its follow-up commit be54b58 removed inference from both plugins and explicitly documented that ENV_TARGET is only set manually.
• Merged #4518 continued the explicit ENV_TARGET tree-shaking path and added emitted-output coverage; the existing fixture still asserts that ENV_TARGET is absent unless configured.
• #4900 adds automatic target selection only in the Modern.js integrations, not in the core Webpack/Rspack plugins.

Because the core plugin behavior and tests intentionally reject inference, this is a documentation-only correction; no code or changeset is included.

Validation: git diff --check passed. The targeted Prettier check was attempted but could not run because this container has Node.js 20.19.2 while the repository's pnpm requires Node.js 22.13+ (the repository CI uses Node.js 24).

Related Issue

Historical context: #4452 (closed). This PR documents the behavior implemented by its follow-up work and does not alter that fix.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the documentation.

[changeset-bot]

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Latest commit: 8636d0a

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