Upgrading docs on new APIs in Versioned

docs/en/04_Changelogs/4.2.0.md

@@ -3,6 +3,7 @@
 ## Overview {#overview}
 
  * Disable session-based stage setting in `Versioned` (see [#1578](https://github.com/silverstripe/silverstripe-cms/issues/1578))
+ * Deprecated `FunctionalTest::useDraftSite()`. You should use querystring args instead for setting stage.
 
 ## Upgrading {#upgrading}
 
@@ -38,3 +39,24 @@ SilverStripe\Versioned\Versioned:
 
 Check our [versioning docs](/developer_guides/model/versioning#controllers)
 for more details.
+
+### New Versioned API
+
+The following methods have been added to [api:SilverStripe\Versioned\Versioned] class:
+
+ * `withVersionedMode()` Allows users to execute a closure which may internally modify
+   the current stage, but will guarantee these changes are reverted safely on return.
+   Helpful when temporarily performing a task in another stage or view mode.
+ * `get_draft_site_secured()` / `set_draft_site_secured()` Enables the explicit toggle
+   of draft site security. By setting this to false, you can expose a draft mode to
+   unauthenticated users. Replaces `unsecuredDraftSite` session var.
+ * `get_default_reading_mode()` / `set_default_reading_mode()` The default reading
+  mode is now configurable. Any non-default reading mode must have querystring args
+  to be visible. This will be the mode choosen for requests that do not have these args.
+  Note that the default mode for CMS is now draft, but is live on the frontend.
+
+A new class [api:SilverStripe\Versioned\ReadingMode] has also been added to assist with
+conversion of the reading mode between:
+ - Reading mode string
+ - DataQuery parameters
+ - Querystring parameters