DOCS Update docs to reflect true config merge priorities

docs/en/02_Developer_Guides/04_Configuration/00_Configuration.md

@@ -153,12 +153,12 @@ the result will be the higher priority false-ish value.
 
 The locations that configuration values are taken from in highest -> lowest priority order are:
 
-- Any values set via a call to Config#merge / Config#set
+- Runtime modifications, ie: any values set via a call to `Config::inst()->update()`
 - The configuration values taken from the YAML files in `_config/` directories (internally sorted in before / after 
 order, where the item that is latest is highest priority)
-- Any static set on an "additional static source" class (such as an extension) named the same as the name of the property
 - Any static set on the class named the same as the name of the property
 - The composite configuration value of the parent class of this class
+- Any static set on an "additional static source" class (such as an extension) named the same as the name of the property
 
 <div class="notice">
 It is an error to have mixed types of the same named property in different locations. An error will not necessarily

docs/en/04_Changelogs/4.0.5.md

@@ -0,0 +1,12 @@
+# 4.0.5
+
+## Notable changes
+
+Fix [#7971](https://github.com/silverstripe/silverstripe-framework/pull/7971) introduces a subtle change of behaviour
+to how `Config` settings are prioritised. In SilverStripe 4 there was a change where `Extension` objects took the
+highest importance when determining Config values; this was deemed to be unexpected and unintuitive as well as making it
+cumbersome and difficult for developers to override module defaults defined in `Extension`s. This change reverts
+behaviour to that of SilverStripe 3 where `Extension` instances are of lowest importance and are used only to set a
+default value. If you rely on your `Extension` or module providing an overriding config value, please move this to yaml.
+
+<!--- Changes below this line will be automatically regenerated -->