minor #7664 Add an explanation about «constraints» validation (chindit, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7664).

Discussion
----------

Add an explanation about «constraints» validation

Added an explanation about the use of «constraints» in Form Classes and the requirement of «ValidatorExtension» if «constraints» keyword is used.

This is currently not explained in doc, nor in Forms, Validation, Custom validation or any other page.

Commits
-------

c9f1b95 Rewords and minor fixes
cb6b2e6 Add an explanation about «constraints» validation

Author: xabbuh

validation.rst

@@ -455,6 +455,29 @@ If you're ever unsure of how to specify an option, either check the API document
 for the constraint or play it safe by always passing in an array of options
 (the first method shown above).
 
+Constraints in Form Classes
+---------------------------
+
+Constraints can be defined while building the form via the ``constraints`` option
+of the form fields::
+
+    public function buildForm(FormBuilderInterface $builder, array $options)
+    {
+        $builder
+            ->add('myField', TextType::class, array(
+                'required' => true,
+                'constraints' => array(new Length(array('min' => 3)))
+            ))
+    }
+
+The ``constraints`` option is only available when adding the ValidatorExtention
+to the formBuilder::
+
+    Forms::createFormFactoryBuilder()
+        ->addExtension(new ValidatorExtension(Validation::createValidator()))
+        ->getFormFactory()
+    ;
+
 .. index::
    single: Validation; Constraint targets