minor #20204 3.0 Upgrade Guide: Describing how to pass data to a form…

… through options resolver (smithandre)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes #20204).

Discussion
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3.0 Upgrade Guide: Describing how to pass data to a form through options resolver

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #18662
| License       | MIT
| Doc PR        |

Enhanced the upgrade guide by adding details regarding passing data to a form through the options resolver.

Commits
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b69bd3f 3.0 Upgrade Guide: Added details describing how to pass data to a form through the options resolver

UPGRADE-3.0.md

@@ -348,6 +348,58 @@ UPGRADE FROM 2.x to 3.0
    $form = $this->createForm(MyType::class);
    ```
 
+ * Passing custom data to forms now needs to be done 
+   through the options resolver. 
+
+    In the controller:
+
+    Before:
+    ```php
+    $form = $this->createForm(new MyType($variable), $entity, array(
+        'action' => $this->generateUrl('action_route'),
+        'method' => 'PUT',
+    ));
+    ```
+    After: 
+    ```php
+    $form = $this->createForm(MyType::class, $entity, array(
+        'action' => $this->generateUrl('action_route'),
+        'method' => 'PUT',
+        'custom_value' => $variable,
+    ));
+    ```
+    In the form type:
+
+    Before:
+    ```php
+    class MyType extends AbstractType
+    {
+        private $value;
+
+        public function __construct($variableValue)
+        {
+            $this->value = $value;
+        }
+        // ...
+    }
+    ```
+
+    After:
+    ```php
+    public function buildForm(FormBuilderInterface $builder, array $options)
+    {
+        $value = $options['custom_value'];
+        // ...
+    }
+
+    public function configureOptions(OptionsResolver $resolver)
+    {
+        $resolver->setDefaults(array(
+            'custom_value' => null,
+        ));
+    }
+    ```
+
  * The alias option of the `form.type_extension` tag was removed in favor of
    the `extended_type`/`extended-type` option.