[CookBook] Update documentation on Generic Form Type Extensions

Author: lemoinem

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 Type ``form``. Thus, a
+form type extension applying to ``form`` would apply to all of these.
+
+A notable exception are the Buttons From Types. Plus, keep in mind that a custom
+Form Type which doesn't inherit neither ``form`` nor ``button`` could always be
+created.