doc: update assert's validation functions

Using `assert.throws()` or `assert.rejects()` in combination with
validation function can be very powerful. Using them by returning
any value besides `true` makes debugging very difficult though
because there's no context or reason why the error validation failed.
Thus, it is recommended to throw an error in such cases instead of
returning anything besides `true`.

doc/api/assert.md

@@ -1210,10 +1210,14 @@ assert.throws(
   () => {
     throw new Error('Wrong value');
   },
-  function(err) {
-    if ((err instanceof Error) && /value/.test(err)) {
-      return true;
-    }
+  (err) => {
+    assert(err instanceof Error);
+    assert(/value/.test(err));
+    // Returning anything from validation functions besides `true` is not
+    // recommended. Doing so results in the caught error being thrown again.
+    // That is mostly not the desired outcome. Throw an error about the specific
+    // validation that failed instead (as done in this example).
+    return true;
   },
   'unexpected error'
 );