Document the "encode" option for Nav items

Nav.php

@@ -66,6 +66,7 @@ class Nav extends Widget
      * - dropDownOptions: array, optional, the HTML options that will passed to the [[Dropdown]] widget.
      * - items: array|string, optional, the configuration array for creating a [[Dropdown]] widget,
      *   or a string representing the dropdown menu. Note that Bootstrap does not support sub-dropdown menus.
+     * - encode: boolean, optional, whether the label will be HTML-encoded. If set, supersedes the $encodeLabels option for only this item.
      *
      * If a menu item is a string, it will be rendered directly without HTML encoding.
      */