add available since v11 for untagged variants

Author: tsnobip

misc_docs/syntax/decorator_unboxed.mdx

@@ -31,6 +31,8 @@ var hi = "hello!";
 
 </CodeTab>
 
+_`@unboxed` for variants with multiple constructors is available since ReScript `11.0.0`._
+
 ### References
 
 * [Untagged variants](variant#untagged-variants)