[Reference][Constraints] Added hint about attaching the expression constraint to a form field

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4
| Fixed tickets | -

As per my discussion with @webmozart a while ago on twitter https://twitter.com/peterrehm/status/434350363940904960 I am now providing the hint about the possibility to add a expression constraint to a form field.

I would provide the complete XML document:

```xml
<!-- src/Acme/DemoBundle/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\DemoBundle\Model\BlogPost">
        <property name="firstName">
            <constraint name="Expression">
                <option name="expression">
                    this.getCategory() in ['php', 'symfony'] or value == false
                </option>
                <option name="message">
                    If this is a tech post, the category should be either php or symfony!
                </option>
            </constraint>
        </property>
    </class>
</constraint-mapping>
```