Update docs after adding support for custom left button on Android (#1014)

docs/adding-buttons-to-the-navigator.md

 ```
 
 ##### Android left button
-On Android, only four button types are supported. You can use them by specifying one of the following ids in your left button definition:
+On Android, four button types are supported by default without the need to provide an icon. You can use them by specifying one of the following ids in your left button definition:
 
 * back
 * cancel
 * accept
 * sideMenu
 
-> Note: It's currently not possibly to change left button icon on Android.
-
 #### Floating Action Button (FAB) - Android only
 Each screen can contain a single Fab which is displayed at the bottom right corner of the screen.