minor #8091 Update js-datepicker usage for new jQuery UI (AdrianBorodziuk, javiereguiluz)

This PR was merged into the 3.3 branch.

Discussion
----------

Update js-datepicker usage for new jQuery UI

Old variable used - "format" - doesn't work anymore in jQuery UI.

Commits
-------

97f3f84 Mention that we are using Bootstrap Datepicker
4d66b50 Update js-datepicker usage for new jQuery UI

Author: javiereguiluz

reference/forms/types/date.rst

@@ -108,7 +108,7 @@ picker:
 
     <script>
         $(document).ready(function() {
-            // configure the bootstrap datepicker
+            // you may need to change this code if you are not using Bootstrap Datepicker
             $('.js-datepicker').datepicker({
                 format: 'yyyy-mm-dd'
             });