minor symfony#31222 [Form] Add Button type back to Form::getClickedButton docblock (Bill Hance)

This PR was merged into the 3.4 branch.

Discussion
----------

[Form] Add Button type back to Form::getClickedButton docblock

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

In commit 56429a6 the `Button` type was removed from the return doc block.

I suspect this was an oversight because, AFAIK, the [recommended way](https://symfony.com/doc/current/form/multiple_buttons.html) to check the name of a clicked button would require a return type of `Button` or `SubmitButton`.

The effect is that our static analysis checks are failing.
```
Call to an undefined method
Symfony\Component\Form\ClickableInterface::getName().
```

Commits
-------

b71d589 Added FormInterface to @return Form::getClickedButton docblock

Author: xabbuh

src/Symfony/Component/Form/Form.php

@@ -89,7 +89,7 @@ class Form implements \IteratorAggregate, FormInterface
     private $submitted = false;
 
     /**
-     * @var ClickableInterface|null The button that was used to submit the form
+     * @var FormInterface|ClickableInterface|null The button that was used to submit the form
      */
     private $clickedButton;
 
@@ -749,7 +749,7 @@ public function isValid()
     /**
      * Returns the button that was used to submit the form.
      *
-     * @return ClickableInterface|null
+     * @return FormInterface|ClickableInterface|null
      */
     public function getClickedButton()
     {