[Cookbook] Fix doc on Generic Form Type Extensions

cookbook/form/create_form_type_extension.rst

@@ -12,18 +12,10 @@ But sometimes, you don't really need to add new field types - you want
 to add features on top of existing types. This is where form type
 extensions come in.
 
-Form type extensions have 2 main use-cases:
-
-#. You want to add a **generic feature to several types** (such as
-   adding a "help" text to every field type);
-#. You want to add a **specific feature to a single type** (such
-   as adding a "download" feature to the "file" field type).
-
-In both those cases, it might be possible to achieve your goal with custom
-form rendering, or custom form field types. But using form type extensions
-can be cleaner (by limiting the amount of business logic in templates)
-and more flexible (you can add several type extensions to a single form
-type).
+It might be possible to achieve your goal with custom form rendering, or custom
+form field types. But using form type extensions can be cleaner (by limiting the
+amount of business logic in templates) and more flexible (you can add several
+type extensions to a single form type).
 
 Form type extensions can achieve most of what custom field types can do,
 but instead of being field types of their own, **they plug into existing types**.
@@ -319,3 +311,15 @@ next to the file field. For example::
 
 When displaying the form, if the underlying model has already been associated
 with an image, you will see it displayed next to the file input.
+
+Generic Form Type Extensions
+----------------------------
+
+Although it is not possible to have a form type extension applying to all form
+types, **most** form types natively available in Symfony
+(:doc:`/reference/forms/types`) inherit from the ``form`` form type. Thus, a
+form type extension applying to ``form`` would apply to all of these.
+
+A notable exception are the ``button`` from types. Plus, keep in mind that a custom
+form type which doesn't inherit neither ``form`` nor ``button`` could always be
+created.

reference/dic_tags.rst

@@ -296,72 +296,8 @@ form.type_extension
 
 **Purpose**: Create a custom "form extension"
 
-Form type extensions are a way for you took "hook into" the creation of
-any field in your form. For example, the addition of the CSRF token is done
-via a form type extension
-(:class:`Symfony\\Component\\Form\\Extension\\Csrf\\Type\\FormTypeCsrfExtension`).
-
-A form type extension can modify any part of any field in your form. To
-create a form type extension, first create a class that implements the
-:class:`Symfony\\Component\\Form\\FormTypeExtensionInterface` interface.
-For simplicity, you'll often extend an
-:class:`Symfony\\Component\\Form\\AbstractTypeExtension` class instead of
-the interface directly::
-
-    // src/Acme/MainBundle/Form/Type/MyFormTypeExtension.php
-    namespace Acme\MainBundle\Form\Type;
-
-    use Symfony\Component\Form\AbstractTypeExtension;
-
-    class MyFormTypeExtension extends AbstractTypeExtension
-    {
-        // ... fill in whatever methods you want to override
-        // like buildForm(), buildView(), finishView(), setDefaultOptions()
-    }
-
-In order for Symfony to know about your form extension and use it, give
-it the ``form.type_extension`` tag:
-
-.. configuration-block::
-
-    .. code-block:: yaml
-
-        services:
-            main.form.type.my_form_type_extension:
-                class: Acme\MainBundle\Form\Type\MyFormTypeExtension
-                tags:
-                    - { name: form.type_extension, alias: field }
-
-    .. code-block:: xml
-
-        <?xml version="1.0" encoding="UTF-8" ?>
-        <container xmlns="http://symfony.com/schema/dic/services"
-            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-            xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">
-
-            <services>
-                <service
-                    id="main.form.type.my_form_type_extension"
-                    class="Acme\MainBundle\Form\Type\MyFormTypeExtension">
-
-                    <tag name="form.type_extension" alias="field" />
-                </service>
-            </services>
-        </container>
-
-    .. code-block:: php
-
-        $container
-            ->register(
-                'main.form.type.my_form_type_extension',
-                'Acme\MainBundle\Form\Type\MyFormTypeExtension'
-            )
-            ->addTag('form.type_extension', array('alias' => 'field'))
-        ;
-
-The ``alias`` key of the tag is the type of field that this extension should
-be applied to. For example, to apply the extension to any form/field, use
-the "form" value.
+For details on creating Form type extensions, read the cookbook article:
+:doc:`/cookbook/form/create_form_type_extension`
 
 .. _reference-dic-type_guesser: