doc: update assert.rejects() docs with a validation function example

Spawned from my own struggle to use in https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1702#note_268452483

PR-URL: #31271
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

Author: MadLittleMods

doc/api/assert.md

@@ -1111,6 +1111,21 @@ if the `asyncFn` fails to reject.
 })();
 ```
 
+```js
+(async () => {
+  await assert.rejects(
+    async () => {
+      throw new TypeError('Wrong value');
+    },
+    (err) => {
+      assert.strictEqual(err.name, 'TypeError');
+      assert.strictEqual(err.message, 'Wrong value');
+      return true;
+    }
+  );
+})();
+```
+
 ```js
 assert.rejects(
   Promise.reject(new Error('Wrong value')),