Button demo: added description

Author: rdworth

demos/button/checkbox.html

@@ -37,7 +37,9 @@
 
 <div class="demo-description">
 
-<p>TODO</p>
+<p>A checkbox is styled as toggle buttons with the button widget. The label element associated with the checkbox is used for the button text.</p>
+
+<p>This demo also demonstrates three checkboxes styled as a button set by calling <code>.buttonset()</code> on a common container.
 
 </div><!-- End demo-description -->