UPGRADING: Document __debugInfo() for enums

Follow-up to php#21425

Author: DanielEScherzer

UPGRADING

@@ -133,6 +133,8 @@ PHP 8.6 UPGRADE NOTES
 - Core:
   . It is now possible to use reference assign on WeakMap without the key
     needing to be present beforehand.
+  . It is now possible to define the `__debugInfo()` magic method on enums.
+    RFC: https://wiki.php.net/rfc/debugable-enums
 
 - Fileinfo:
   . finfo_file() now works with remote streams.