doc: include it in util.md

Author: AugustinMauroy

doc/api/util.md

@@ -3807,6 +3807,12 @@ Node.js modules. The community found and used it anyway.
 It is deprecated and should not be used in new code. JavaScript comes with very
 similar built-in functionality through [`Object.assign()`][].
 
+An automated migration is available ([source](https://github.com/nodejs/userland-migrations/tree/main/recipes/util-extend-to-object-assign)):
+
+```bash
+npx codemod@latest @nodejs/util-extend-to-object-assign
+```
+
 ### `util.isArray(object)`
 
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