Documented the new csrf_token field prefix

Author: javiereguiluz

security/csrf.rst

@@ -108,6 +108,15 @@ this can be customized on a form-by-form basis::
         // ...
     }
 
+You can also customize the rendering of the CSRF form field creating a custom
+:doc:`form theme </form/form_themes>` and using ``csrf_token`` as the prefix of
+the field (e.g. define ``{% block csrf_token_widget %} ... {% endblock %}`` to
+customize the entire form field contents).
+
+.. versionadded:: 4.3
+
+    The ``csrf_token`` form field prefix was introduced in Symfony 4.3.
+
 CSRF Protection in Login Forms
 ------------------------------