Update documentation regarding pages dynamic id

Author: marcosvafilho

docs/ui-in-pure-ruby/overview.md

@@ -144,6 +144,16 @@ end
 ```
 {% endcode %}
 
+Both the controller and action names will be dynamically added to the `id` attribute of the page's root element, allowing CSS rules to directly target specific pages, and tests to easily locate the page's content.
+
+For example, the above controller code will result in the following HTML markup:
+
+```markup
+  <div id="matestack-page-some-controller-overview" class="matestack-page-root">
+    <!-- page content -->
+  </div>
+```
+
 Learn more about Pages:
 
 {% page-ref page="pages/" %}