Clarify that Transition Actions can be asynchronous

[yotatameguchi-boop]

The troubleshooting section says that the function passed to startTransition must be synchronous, even though the page documents async Actions and demonstrates startTransition(async () => { ... }).

Replace that restriction with the constraint demonstrated by the adjacent example: updates in a later setTimeout callback are not marked as Transitions. Explicitly acknowledge that Actions can be asynchronous. The following section continues to explain the separate requirement to wrap state updates after await in another startTransition.

Validation:

• Confirmed consistency with the action parameter description and the existing async Action examples.
• git diff --check passes.
• Verified that code blocks, links, heading anchors, and line count are unchanged.

This is a prose-only change. The site build and browser preview were not run.

[github-actions]

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