Currently, the basic examples of constraints like GreaterThan don't use the short version for default options:

class Person
{
    /**
     * @Assert\GreaterThan(
     *     value = 18
     * )
     */
    protected $age;
}

The proper (short) way of writing this would be:

class Person
{
    /**
     * @Assert\GreaterThan(18)
     */
    protected $age;
}

I think the "Basic Usage" examples should be fixed in this direction. In addition, a new section "Message Customization" should be added that shows how to write the constraint when customizing the message:

class Person
{
    /**
     * @Assert\GreaterThan(
     *     value = 18,
     *     message = "Restricted to persons older than 18 years."
     * )
     */
    protected $age;
}

The above applies for all configuration styles (annotations, PHP, XML and YAML).