Document proper usage of setDefaultOptions

docs/docs/styling.md

@@ -32,6 +32,8 @@ export default class StyledScreen extends Component {
 ## Enabling persistent styling properties
 In v2 we added `setDefaultOptions` API for styles that should be applied on all components.
 
+> `setDefaultOptions` Does not update options of existing component, therefore it should be called before `setRoot`
+
 ```js
 Navigation.setDefaultOptions({
   topBar: {