docs(config): add TS callback literal widening note

[Vishnuuuu24]

Summary

• add a docs note for TypeScript users about callback-based defineConfig literal widening
• document practical workarounds (as const or explicit callback return type) for overload errors in async/sync callback config

Why

Issue #21684 reports that in callback-based config, some nested string literals can be widened by TypeScript and trigger overload errors. This PR documents the existing TypeScript limitation and recommended workaround in the Async Config section.

Changes

• update docs/config/index.md under Async Config with a short note and guidance

Validation

• pnpm run docs-build

Refs #21684

[Copilot]

Pull request overview

This PR adds documentation guidance for TypeScript users encountering literal widening issues in callback-based defineConfig. The change addresses issue #21684 where users experience TypeScript overload errors when setting options like minify: 'terser' in async config callbacks.

Changes:

• Added a note after the Async Config section explaining TypeScript literal widening in callback-based configs and providing two workarounds (as const or explicit return type annotation)