Clarify error handling for standalone startTransition

[aurorascharff]

Summary

• document the pending-state, component-association, and error-handling differences between the standalone startTransition function and the function returned by useTransition
• clarify that errors from a standalone Action are reported as uncaught, while errors from an Action passed to the hook-returned function can be handled by the component's nearest Error Boundary
• note that the standalone function is not a Hook and can also be called outside components

Validation

• yarn eslint src/content/reference/react/startTransition.md src/content/reference/react/useTransition.md
• yarn lint-heading-ids
• yarn tsc
• git diff --check

[github-actions]

Size changes

Details

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