Update form_customization.rst

Follow up of #13684

This is meant to be a caution box and a link to https://symfony.com/doc/4.4/form/bootstrap4.html#error-messages - if I didn't get it right, please fix it :-)

Author: ThomasLandauer

form/form_customization.rst

@@ -255,6 +255,11 @@ Renders any errors for the given field.
     {# render any "global" errors not associated to any form field #}
     {{ form_errors(form) }}
 
+.. caution::
+
+    If you're using the Bootstrap 4 form theme, `form_errors()` is already included
+    in `form_label()`, see ":ref:`reference-forms-bootstrap-error-messages`"
+
 .. _reference-forms-twig-widget:
 
 form_widget(form_view, variables)