[Cookbook] Specify how to create a generic form type extension in the appropriate doc (got it from reference/dic_tags.html#form-type-extension)

Author: lemoinem

cookbook/form/create_form_type_extension.rst

@@ -129,6 +129,9 @@ The ``alias`` key of the tag is the type of field that this extension should
 be applied to. In your case, as you want to extend the ``file`` field type,
 you will use ``file`` as an alias.
 
+If you want to apply the extension to all field types, you can specify ``form``
+as alias and value returned by ``getExtendedType()``.
+
 Adding the extension Business Logic
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