Add info about callback in options resolver

Author: marekkalnik

components/options_resolver.rst

@@ -305,6 +305,26 @@ There is also an
 method, which you can use if you want to add an allowed value to the previously
 set allowed values.
 
+If you need to add some more logic to the value validation process you can pass a callable
+as an allowed value::
+
+    // ...
+    protected function setDefaultOptions(OptionsResolverInterface $resolver)
+    {
+        // ...
+
+        $resolver->setAllowedValues(array(
+            'transport' => function($value) {
+                return strpos($value, 'mail') !== false;
+            }
+        ));
+    }
+
+Note that using this together with addAllowedValues will not work.
+
+.. versionadded:: 2.5
+    The callback support for allowed values was added in Symfony 2.5.
+
 Configure allowed Types
 ~~~~~~~~~~~~~~~~~~~~~~~