added missing documentation (#4223)

site/docs/extensions/filter-control.md

@@ -162,3 +162,7 @@ Dependence if you use the datepicker option: [bootstrap-datepicker](https://gith
 ### triggerSearch
 
 * Trigger manually the search action
+
+### clearFilterControl
+
+* Clear all the controls added by this plugin (similar to filterShowClear option).