feature #3818 [Form customization] added block_name example. (aitboudad)

This PR was merged into the 2.3 branch.

Discussion
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[Form customization] added block_name example.

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

Commits
-------

512d5eb [Form customization] fixed minor typos
e423118 [Form customization] added block_name example.

Author: weaverryan

cookbook/form/form_customization.rst

@@ -660,6 +660,23 @@ field whose *id* is ``product_name`` (and name is ``product[name]``).
    ``ProductType`` equates to ``product``). If you're not sure what your
    form name is, just view the source of your generated form.
 
+   If you want to change the ``product`` or ``name`` portion of the block
+   name ``_product_name_widget`` you can set the ``block_name`` option in your
+   form type::
+
+        use Symfony\Component\Form\FormBuilderInterface;
+
+        public function buildForm(FormBuilderInterface $builder, array $options)
+        {
+            // ...
+
+            $builder->add('name', 'text', array(
+                'block_name' => 'custom_name',
+            ));
+        }
+
+   Then the block name will be ``_product_custom_name_widget``.
+
 You can also override the markup for an entire field row using the same method:
 
 .. configuration-block::