Update doc for Navigation api , goback method. Following up on #408

Author: Morgan

docs/api/mixins/Navigation.md

@@ -36,13 +36,19 @@ this.replaceWith('/users/10?showAge=true');
 ### `goBack()`
 
 Programmatically go back to the last route and remove the most recent
-entry from the browser history.
+entry from the browser history. Return false if it's the first entry in the app history, true otherwise.
 
 #### Example
 
 ```js
 this.goBack();
 ```
+If you want to make sure there is an history entry
+```js
+if (!this.goBack()) {
+  this.transitionTo('/otherpage')
+}
+```
 
 ### `makePath(routeName, params, query)`