docs: Add default server-side cacheTime to migration docs

docs/src/pages/guides/migrating-to-react-query-4.md

@@ -282,7 +282,7 @@ It was possible to change the logger globally by calling `setLogger`. In v4, tha
 + const queryClient = new QueryClient({ logger: customLogger })
 ```
 
-### Undefined is an illegale cache value for successful queries
+### Undefined is an illegal cache value for successful queries
 
 In order to make bailing out of updates possible by returning `undefined`, we had to make `undefined` an illegal cache value. This is in-line with other concepts of react-query, for example, returning `undefined` from the [initialData function](guides/initial-query-data#initial-data-function) will also _not_ set data.
 
@@ -301,6 +301,14 @@ This is now disallowed on type level; at runtime, `undefined` will be transforme
 
 As of v4, React Query is optimized for modern browsers. We have updated our browserslist to produce a more modern, performant and smaller bundle. You can read about the requirements [here](../installation#requirements).
 
+### No _default_ manual Garbage Collection server-side
+
+In v3, React Query would cache query results for a default of 5 minutes, then manually garbage collect that data. This default was applied to server-side React Query as well.
+
+This lead to high memory consumption and hanging processes waiting for this manual garbage collection to complete. In v4, by default the server-side `cacheTime` is now set to `Infinity` effectively disabling manual garbage collection (the NodeJS process will clear everything once a request is complete).
+
+This change only impacts users of server-side React Query, such as with Next.js. If you are setting a `cacheTime` manually this will not impact you (although you may want to mirror behavior).
+
 ## New Features 🚀
 
 ### Proper offline support

docs/src/pages/guides/ssr.md

@@ -202,7 +202,9 @@ This refetching of stale queries is a perfect match when caching markup in a CDN
 
 ### High memory consumption on server
 
-In case you are creating the `QueryClient` for every request, React Query creates the isolated cache for this client, which is preserved in memory for the `cacheTime` period (which defaults to 5 minutes). That may lead to high memory consumption on server in case of high number of requests during that period.
+In case you are creating the `QueryClient` for every request, React Query creates the isolated cache for this client, which is preserved in memory for the `cacheTime` period. That may lead to high memory consumption on server in case of high number of requests during that period.
+
+On the server, `cacheTime` defaults to `Infinity` which disables manual garbage collection and will automatically clear memory once a request has finished. If you are explicitly setting a non-Infinity `cacheTime` then you will be responsible for clearing the cache early.
 
 To clear the cache after it is not needed and to lower memory consumption, you can add a call to [`queryClient.clear()`](../reference/QueryClient#queryclientclear) after the request is handled and dehydrated state has been sent to the client.
 

docs/src/pages/guides/testing.md

@@ -88,7 +88,7 @@ const queryClient = new QueryClient({
 
 ## Set cacheTime to Infinity with Jest
 
-`cacheTime` is set to 5 minutes by default. It means that the cache garbage collector timer will be triggered every 5 minutes. If you use Jest, you can set the `cacheTime` to `Infinity` to prevent "Jest did not exit one second after the test run completed" error message.
+If you use Jest, you can set the `cacheTime` to `Infinity` to prevent "Jest did not exit one second after the test run completed" error message. This is the default behavior on the server, and is only necessary to set if you are explicitly setting a `cacheTime`.
 
 ## Testing Network Calls