hint for mapping request payload error messages

Co-authored-by: Oskar Stark <oskarstark@googlemail.com>

Author: dbu

controller.rst

@@ -502,6 +502,14 @@ payload formats::
         // ...
     }
 
+.. tip::
+
+    If you build a JSON API, make sure to declare your route as using the JSON
+    :ref:`format <routing-format-parameter>`. This will make the error handling
+    output a JSON response in case of validation errors, rather than an HTML page::
+
+        #[Route('/dashboard', name: 'dashboard', format: 'json')]
+
 Make sure to install `phpstan/phpdoc-parser`_ and `phpdocumentor/type-resolver`_
 if you want to map a nested array of specific DTOs::