Add upgrading note regarding merge of RFC for new DOMParentNode/DOMChildNode APIs.

Author: beberlei

UPGRADING

@@ -415,6 +415,10 @@ PHP 8.0 UPGRADE NOTES
   . Added DateTime::createFromInterface() and
     DateTimeImmutable::createFromInterface().
 
+- dom:
+  . Introduce DOMParentNode and DOMChildNode with new traversal and manipulation APIs
+    RFC: https://wiki.php.net/rfc/dom_living_standard_api
+
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 3. Changes in SAPI modules
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