add link to blog post about untagged variants

Author: tsnobip

misc_docs/syntax/decorator_unboxed.mdx

@@ -35,5 +35,6 @@ _`@unboxed` for variants with multiple constructors is available since ReScript
 
 ### References
 
-* [Untagged variants](variant#untagged-variants)
+* [Untagged variants doc](variant#untagged-variants)
+* [Blog post introducing untagged variants](blog/improving-interop)
 * [Unboxed record](/docs/manual/v10.0.0/unboxed)