docs: migration guide for window focus changes

Author: TkDodo

docs/react/guides/migrating-to-v5.md

@@ -162,6 +162,10 @@ useQuery<number, string>({
 
 Since the only supported syntax now is the object syntax, this rule is no longer needed
 
+### Window focus refetching no longer listens to the `focus` event
+
+The `visibilitychange` event is used exclusively now. This is possible because we only support browsers that support the `visibilitychange` event. This fixes a bunch of issues [as listed here](https://github.com/TanStack/query/pull/4805).
+
 ### No longer using `unstable_batchedUpdates` as the batching function in React and React Native
 
 Since the function `unstable_batchedUpdates` is noop in React 18, it will no longer be automatically set as the batching function in `react-query`.
@@ -171,8 +175,8 @@ If your framework supports a custom batching function, you can let TanStack Quer
 For example, this is how the batch function is set in `solid-query`:
 
 ```ts
-import { notifyManager } from '@tanstack/query-core' 
-import { batch } from 'solid-js' 
-  
-notifyManager.setBatchNotifyFunction(batch) 
+import { notifyManager } from '@tanstack/query-core'
+import { batch } from 'solid-js'
+
+notifyManager.setBatchNotifyFunction(batch)
 ```