Clarify StackNavigator inside DrawerNavigator gotchas

Moved from https://github.com/react-navigation/react-navigation/issues/3890

Using multiple Navigators inside a DrawerNavigator, where the subnavigators shared a route name, led to unexpected (undocumented) behavior

Context

I was an early user of @expo/ex-navigation, and have recently been upgrading to react-navigation. I came across some faults in either the DrawerNavigation documentation or in the general mental framework of react-navigation.

Current Behavior

When nesting navigators inside a DrawerNavigator, duplicated route names behave unexpectedly (in my mental model).
Expo Snack repro

const WidgetStack = StackNavigator({
  Widget: {
    screen: WidgetScreen,
  },
  // This is very easy to get wrong, it is not clear in the Drawer documentation
  // that this route name should be distinct.
  Settings: {
    screen: SettingsScreen,
  },
});

const SettingsStack = StackNavigator({
  Settings: {
    screen: SettingsScreen,
  },
});

const DrawerApp = DrawerNavigator({
  Widget: {
    screen: WidgetStack,
  },
  Settings: {
    screen: SettingsStack,
  },
}, {
  initialRouteName: 'Widget',
});

When tapping on the "Settings" drawer item, I expected the currently selected "tab" in the drawer to be "Settings", I expected no routes to be "pushed" onto my stack navigator. Instead, the "widget" tab stays selected, and a settings route is pushed onto the WidgetStack.

There were no pieces of documentation to warn me of this behavior, and it was actually very difficult to even tell this is what was happening (it was much easier when I decided to build the minimal repro).

My current mental model

My mental model for react-navigation was previously that each nested navigator was a "subtree". With that model, I saw no issue with a duplicated route name.

               DrawerApp
	      /         \
	  WidgetStack    SettingsStack
          |     |	       |
	  /      \           Settings
	Widget   Settings

I'm not here to debate whether this is the correct model, but a simple piece of documentation would have saved me about an entire day :(

I would be happy to submit a PR to address this, but it appears with the 2.0 push, this might be out of date, and furthermore - I am not sure if my current model is correct enough to even write docs.

DrawerNavigator とネストされた StackNavigator のドキュメントから始め、リンクされている Expo Snack と参照されている 2.0 push を確認して、その動作が現在も当てはまるかどうかを判断します。関連するドキュメントを更新し、重複した route-name に関する落とし穴とネストのモデルを説明するとともに、何が完了したものとしてドキュメント化されているかを示す現在の例または再現を記載します。