Added segmented content fallback

Author: AndyButland

17/umbraco-cms/fundamentals/setup/upgrading/version-specific/README.md

@@ -89,6 +89,14 @@ The color picker property editor used for the built-in approved color Data Type
 
 For more details on this update see the following PR: [#19430](https://github.com/umbraco/Umbraco-CMS/pull/19430).
 
+**Segmented Content Fallback**
+
+The template and delivery API output segmented properties will perform an explicit fallback to the default segment, if they do not have a value.
+
+In earlier versions, if you created a segmented version of a document, you had to complete every property. This made segments quite an editorial burden unless the behavior was customized. With the new behavior, segmented content only needs properties that need a segmented value need to be completed.
+
+For more details on this update see the following PR: [#20309](https://github.com/umbraco/Umbraco-CMS/pull/20309).
+
 **Removal of Extension Methods**
 
 Extension and public helper methods, unused in Umbraco and obsolete in previous versions, have been removed.