Add dashboard search placeholder to describe search patterns

Since ^ and $ for searching from start and to end is supported, let's
tell the user about it.

powerdnsadmin/templates/dashboard.html

@@ -174,6 +174,9 @@
         $(id).DataTable({
             "paging" : true,
             "lengthChange" : true,
+            language: {
+                searchPlaceholder: "Use ^ and $ for start and end",
+            },
             "searching" : true,
             "ordering" : true,
             "columnDefs": [