Clarify sync and async Transition callbacks

Author: aurorascharff

src/content/reference/react/useTransition.md

@@ -1684,7 +1684,7 @@ startTransition(() => {
 });
 ```
 
-The Action you pass to `startTransition` can be asynchronous. You can't mark an update as a Transition like this:
+The function you pass to `startTransition` can be synchronous or asynchronous. However, updates scheduled in a `setTimeout` callback aren't marked as Transitions:
 
 ```js
 startTransition(() => {