docs: clarify onMount with non-synchronous handlers (#5053)

Author: antony

site/content/docs/03-run-time.md

@@ -50,6 +50,8 @@ If a function is returned from `onMount`, it will be called when the component i
 </script>
 ```
 
+> This behaviour will only work when the function passed to `onMount` *synchronously* returns a value. `async` functions always return a `Promise`, and as such cannot *synchronously* return a function.
+
 #### `beforeUpdate`
 
 ```js