[forms] Missing XML validation

Author: weaverryan

book/forms.rst

@@ -269,6 +269,11 @@ number:
  <property name="name">
  <constraint name="NotBlank" />
  </property>
+ <property name="price">
+ <constraint name="Min">
+ <value>0</value>
+ </constraint>
+ </property>
  </class>
 
  .. code-block:: php-annotations