minor #13685 Update form_customization.rst (ThomasLandauer)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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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 :-)

Commits
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50dfc90 Update form_customization.rst

Author: wouterj

form/bootstrap4.rst

@@ -77,6 +77,8 @@ If you prefer to apply the Bootstrap styles on a form to form basis, include the
         {{ form(form) }}
     {% endblock %}
 
+.. _reference-forms-bootstrap-error-messages:
+
 Error Messages
 --------------
 

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::
+
+    In 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)