feature #4456 New validation API usage in Class Constraint Validator (skwi)

This PR was squashed before being merged into the 2.5 branch (closes #4456).

Discussion
----------

New validation API usage in Class Constraint Validator

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5
| Fixed tickets |

The code in the documentation about Class Constraint Validator was still using the deprecated validation API

Commits
-------

e431fdb New validation API usage in Class Constraint Validator

Author: wouterj

cookbook/validation/custom_constraint.rst

@@ -230,12 +230,20 @@ With this, the validator ``validate()`` method gets an object as its first argum
         public function validate($protocol, Constraint $constraint)
         {
             if ($protocol->getFoo() != $protocol->getBar()) {
+                // If you're using the new 2.5 validation API (you probably are!)
+                $this->context->buildViolation($constraint->message)
+                    ->atPath('foo')
+                    ->addViolation();
+
+                // If you're using the old 2.4 validation API
+                /*
                 $this->context->addViolationAt(
                     'foo',
                     $constraint->message,
                     array(),
                     null
                 );
+                */
             }
         }
     }