docs: add more detail to assert recommendation

CONTRIBUTING.md

@@ -56,7 +56,12 @@ tests, ideally reducing dependencies on third party libraries.
 
 For tests, using the standard
 library [assert](https://nodejs.org/docs/latest-v18.x/api/assert.html) is
-preferred.
+preferred. The library provides a strict and a legacy mode; please use the
+strict mode as shown below:
+
+```js
+const assert = require('node:assert/strict');
+```
 
 If you want to use a third party package, help us to understand if you have
 requirements not fulfilled by core libraries in the description of your pull