Clarify that Transition Actions can be asynchronous (#8696)

* docs: clarify that Transition Actions can be asynchronous

* Keep async Action clarification focused

* Clarify sync and async Transition callbacks

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Co-authored-by: Aurora Scharff <aurora.sofie@gmail.com>

Author: yotatameguchi-boop

src/content/reference/react/useTransition.md

@@ -1684,7 +1684,7 @@ startTransition(() => {
 });
 ```
 
-The function you pass to `startTransition` must be synchronous. 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(() => {