Fix according to PR comments

Author: marekkalnik

components/options_resolver.rst

@@ -305,7 +305,11 @@ 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
+.. versionadded:: 2.5
+
+    The callback support for allowed values was introduced in Symfony 2.5.
+
+If you need to add some more logic to the value validation process, you can pass a callable
 as an allowed value::
 
     // ...
@@ -315,15 +319,14 @@ as an allowed value::
 
         $resolver->setAllowedValues(array(
             'transport' => function($value) {
-                return strpos($value, 'mail') !== false;
-            }
+                return false !== strpos($value, 'mail');
+            },
         ));
     }
 
-Note that using this together with addAllowedValues will not work.
+.. caution::
 
-.. versionadded:: 2.5
-    The callback support for allowed values was added in Symfony 2.5.
+    Note that using this together with ``addAllowedValues`` will not work.
 
 Configure allowed Types
 ~~~~~~~~~~~~~~~~~~~~~~~