minor #5227 [Cookbook] Fix doc on Generic Form Type Extensions (lemoinem)

This PR was squashed before being merged into the 2.3 branch (closes #5227).

Discussion
----------

[Cookbook] Fix doc on Generic Form Type Extensions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >=2.3
| Fixed tickets | N/A

Follow up on #5154 (Sorry the first one didn't use the PR Template BTW) and symfony/symfony#14456.

I cleaned up the reference on DIC Tags, I think referring to the cookbook article prevents duplicating documentation (and taking the risk it will get out of date or inconsistent later on, as it did).

Then I removed the mention of Generic FTE at the top of the cookbook article. Since the feature doesn't exists anymore, I think it's better to leave it aside. I simply mentioned why it's impossible in a short last paragraph at the end of the CB entry.

I kept both commit unsquashed because they seemed to address different enough concerns. But I will squash them upon request if you think it's necessary.

Commits
-------

a6bc497 [Cookbook] Fix doc on Generic Form Type Extensions

Author: xabbuh

cookbook/form/create_form_type_extension.rst

@@ -14,16 +14,15 @@ 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).
+   as adding a "download" feature to the "file" field type);
+#. You want to add a **generic feature to several types** (such as
+   adding a "help" text to every "input text"-like 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 +318,19 @@ 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
+----------------------------
+
+You can modify several form types at once by specifying their common parent
+(:doc:`/reference/forms/types`). For example, several form types natively
+available in Symfony inherit from the ``text`` form type (such as ``email``,
+``search``, ``url``, etc.). A form type extension applying to ``text``
+(i.e. whose ``getExtendedType`` method returns ``text``) would apply to all of
+these form types.
+
+In the same way, since **most** form types natively available in Symfony inherit
+from the ``form`` form type, a form type extension applying to ``form`` would
+apply to all of these.  A notable exception are the ``button`` form types. Plus,
+keep in mind that a custom form type which 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: