docs(datepicker): add note about 'modelDateFormat' and 'dateType'

Closes mgcrea#1851

Author: vmlf01

src/datepicker/docs/datepicker.demo.html

@@ -155,7 +155,8 @@ <h4>This module supports exotic placement options!</h4>
           <td>string</td>
           <td>null</td>
           <td>
-            <p>Model format of your date, leverages <a href="http://docs.angularjs.org/api/ng.filter:date">ng.filter:date</a>.</p>
+            <p>Model format of your date, leverages <a href="http://docs.angularjs.org/api/ng.filter:date">ng.filter:date</a>. You should also set dateType to <code>string</code>
+            to use this option.</p>
           </td>
         </tr>
         <tr>