NavigationItemForm: Provide a helpful hint how to input external urls

refs #5600

application/forms/Navigation/NavigationItemForm.php

@@ -52,7 +52,9 @@ class NavigationItemForm extends Form
                 'allowEmpty'    => true,
                 'label'         => $this->translate('Url'),
                 'description'   => $this->translate(
-                    'The url of this navigation item. Leave blank if you only want the name being displayed'
+                    'The url of this navigation item. Leave blank if you only want the'
+                    . ' name being displayed. For external urls, make sure to prepend'
+                    . ' an appropriate protocol identifier (e.g. http://example.tld)'
                 )
             )
         );