Add missing rendering of form help block.

src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_3_layout.html.twig

@@ -114,6 +114,7 @@
     <div class="form-group{% if (not compound or force_error|default(false)) and not valid %} has-error{% endif %}">
         {{- form_label(form) }} {# -#}
         {{ form_widget(form, widget_attr) }} {# -#}
+        {{- form_help(form) -}}
         {{ form_errors(form) }} {# -#}
     </div> {# -#}
 {%- endblock form_row %}