Imporve docs for the prop "navigationBar" in Navigator

Summary:The doc of props "navigationBar" in Navigator is not  detailed enough. I make an improvement to it.
Closes #6615

Differential Revision: D3102065

fb-gh-sync-id: da96e3c422e053d0a8203bbd160ea10ed590878a
fbshipit-source-id: da96e3c422e053d0a8203bbd160ea10ed590878a

Author: starkwang

Libraries/CustomComponents/Navigator/Navigator.js

@@ -236,8 +236,9 @@ var Navigator = React.createClass({
     onDidFocus: PropTypes.func,
 
     /**
-     * Optionally provide a navigation bar that persists across scene
-     * transitions
+     * Optionally provide a component as navigation bar that persists across scene
+     * transitions. The component will receive two props: `navigator` and `navState`.
+     * It will be rerendered when the routes change.
      */
     navigationBar: PropTypes.node,