Added information about the new date handling in the comparison constraints and Range

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11673)
| Applies to    | 2.6+
| Fixed tickets | -
I prefer to have fully working XML examples:

```xml
<!-- src/Acme/OrderBundle/Resources/config/validation.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping http://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">

    <class name="Acme\OrderBundle\Entity\Order">
        <property name="deliveryDate">
            <constraint name="GreaterThan">today</constraint>
        </property>
    </class>
</constraint-mapping>
```

This also applies to the other XML code blocks below.