Add user_create form name explanation

Author: Arkalo2

forms.rst

@@ -989,6 +989,9 @@ The extra fields can be injected like this:
 
 Here, the referral code is an extra field injected at view level.
 
+The field name is composed of form ``user_create`` and the field name ``referralCode``. 
+It's automatically generated from the form class name. You can :ref:`override it <changing-the-form-name>` 
+
 You can get the referral code via ``getExtraData``::
 
     $extraData = $form->getExtraData();