Better ErrorPage upgrading guide (#7066)

See #4149

docs/en/04_Changelogs/4.0.0.md

@@ -883,9 +883,19 @@ public function augmentSQL(SQLSelect $query, DataQuery $dataQuery = null)
 }
 ```
 
-#### Install the ErrorPage module
+#### Moved ErrorPage into a new module
 
-ErrorPage has been moved to a separate module to allow for alternative approaches to managing error responses. Please install [silverstripe/errorpage](http://addons.silverstripe.org/add-ons/silverstripe/errorpage) to preserve the 3.x functionality.
+ErrorPage has been moved to a separate [silverstripe/errorpage module](http://addons.silverstripe.org/add-ons/silverstripe/errorpage)
+to allow for alternative approaches to managing error responses.
+The module is installed by default on new projects, but needs to be added to existing projects
+to preserve functionality on the existing "Page not found" and "Server error" pages in the CMS.
+
+    composer require silverstripe/errorpage
+
+Alternatively you can implement your own `onBeforeHTTPError()` handling to present custom errors.
+By default, SilverStripe will display a plaintext "not found" message when the module isn't installed.
+Check the [module upgrading guide](http://addons.silverstripe.org/add-ons/silverstripe/errorpage)
+for more configuration API changes on the `ErrorPage` class.
 
 #### Upgrading asset web.config, .htaccess, or other server configuration