minor #11905 Added example for direct form submit with validation (patrickbussmann)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #11905).

Discussion
----------

Added example for direct form submit with validation

See symfony/symfony#25331.

I had the same issue.
My registration form is filled via an API call and this call contains only filled fields.
But when the user is missing all fields its an empty array and then the username and email will not be validated.
After I found out that this is the behaviour I want to share this knowledge ;-)

Commits
-------

c81d30a Added example for direct form submit with validation

Author: javiereguiluz

form/direct_submit.rst

@@ -89,3 +89,8 @@ method, pass the submitted data directly to
     "PATCH" method, the validation extension will only handle the submitted
     fields. If the underlying data needs to be validated, this should be done
     manually, i.e. using the validator.
+    
+    When you need validation for some fields you can extend your data array with the required ones so that they will be validated.
+    i.e.::
+    
+        $form->submit(array_merge(['email' => null, 'username' => null], $request->request->all()), false);