Update override.rst

The word 'or' is clear. / not so much.

Author: Francisco Corrales Morales

bundles/override.rst

@@ -39,7 +39,7 @@ Services & Configuration
 
 If you want to modify service definitions of another bundle, you can use a compiler
 pass to change the class of the service or to modify method calls. In the following
-example, the implementing class for the ``original-service-id`` is changed to 
+example, the implementing class for the ``original-service-id`` is changed to
 ``Acme\DemoBundle\YourService``::
 
     // src/Acme/DemoBundle/DependencyInjection/Compiler/OverrideServiceCompilerPass.php
@@ -72,17 +72,16 @@ associations. Learn more about this feature and its limitations in
 Forms
 -----
 
-In order to override a form type, it has to be registered as a service (meaning
-it is tagged as ``form.type``). You can then override it as you would override any
-service as explained in `Services & Configuration`_. This, of course, will only
-work if the type is referred to by its alias rather than being instantiated,
-e.g.::
+Form types are referred to by their fully-qualified class name::
 
-    $builder->add('name', 'custom_type');
+    $builder->add('name', CustomType::class);
 
-rather than::
+This means that you cannot override this by creating a sub-class of ``CustomType``
+and registering it as a service and tagging it with ``form.type`` (you *could*
+do this in an earlier version).
 
-    $builder->add('name', new CustomType());
+Instead, you should use a "form type extension" to modify the existing form type.
+For more information, see :doc:`/form/create_form_type_extension`.
 
 .. _override-validation:
 
@@ -93,7 +92,7 @@ Symfony loads all validation configuration files from every bundle and
 combines them into one validation metadata tree. This means you are able to
 add new constraints to a property, but you cannot override them.
 
-To override this, the 3rd party bundle needs to have configuration for
+To overcome this, the 3rd party bundle needs to have configuration for
 :doc:`validation groups </validation/groups>`. For instance, the FOSUserBundle
 has this configuration. To create your own validation, add the constraints
 to a new validation group: