Add note to tag handling page

Author: seekuehe

docs/xml-and-html-handling/tag-handling-v2.mdx

@@ -13,6 +13,10 @@ Tag handling v2 uses an algorithm that balances both translation quality and for
 
 V2 enforces strict XML parsing and will return an error if invalid XML is submitted.
 
+<Note>
+  Customers who had not used tag handling prior to 2025-12-01 default to v2. All other customers default to v1.
+</Note>
+
 ## Usage Examples
 
 ### Translation with v2