add mention of required classes to horizontal forms

docs/templates/pages/base-css.mustache

@@ -909,7 +909,7 @@
         </fieldset>
       </form>
       <h3>{{_i}}Example markup{{/i}}</h3>
-      <p>{{_i}}Given the above example form layout, here's the markup associated with the first input and control group.{{/i}}</p>
+      <p>{{_i}}Given the above example form layout, here's the markup associated with the first input and control group. The <code>.control-group</code>, <code>.control-label</code>, and <code>.controls</code> classes are all required for styling.{{/i}}</p>
 <pre class="prettyprint linenums">
 &lt;form class="form-horizontal"&gt;
   &lt;fieldset&gt;