Adding label_html for asterisk

Closes #14915

Author: ThomasLandauer

form/bootstrap4.rst

@@ -122,6 +122,13 @@ Symfony Form ``RadioType`` and ``CheckboxType`` by adding some classes to the la
 * For a `custom checkbox`_, use ``checkbox-custom``;
 * For having a `switch instead of a checkbox`_, use ``switch-custom``.
 
+Appending an Asterisk to Required Fields
+----------------------------------------
+
+Since form errors are rendered *inside* the ``<label>``, you cannot use CSS ``:after``
+to append an asterisk to the label, cause it would be displayed after the error message.
+Use 
+
 .. code-block:: twig
 
     {{ form_row(form.myRadio, {label_attr: {class: 'radio-custom'} }) }}