Update documentation on Title. (react-navigation#2092)

Author: tomasreimers

docs/api/navigators/StackNavigator.md

@@ -94,7 +94,7 @@ Visual options:
 
 #### `title`
 
-String that can be used as a fallback for `headerTitle` and `tabBarLabel`
+String that can be used as a fallback for `headerTitle`. Additionally, will be used as a fallback for `tabBarLabel` (if nested in a TabNavigator) or `drawerLabel` (if nested in a DrawerNavigator). 
 
 #### `header`