nav: Cut more unused action creators; document shadow navigation.

These two aren't called, but we *do* go to the routes -- by the one
other way we do so besides invoking nav action creators, which is to
return them from `navReducer`.  Mention those here -- as well as the
two others that we can navigate to that way -- for cross-reference.

src/nav/navActions.js

@@ -20,6 +20,12 @@ export const navigateBack = () => (dispatch: Dispatch, getState: GetState): Navi
 export const navigateToChat = (narrow: Narrow): NavigateAction =>
   NavigationActions.navigate({ routeName: 'chat', params: { narrow } });
 
+// Other stack routes reached through `navReducer`:
+//    NavigationActions.navigate({ routeName: 'loading' });
+//    NavigationActions.navigate({ routeName: 'welcome' });
+//    NavigationActions.navigate({ routeName: 'account' });
+//    NavigationActions.navigate({ routeName: 'main' });
+
 export const navigateToUsersScreen = (): NavigateAction =>
   NavigationActions.navigate({ routeName: 'users' });
 
@@ -52,9 +58,6 @@ export const navigateToRealmScreen = (realm?: string): NavigateAction =>
 export const navigateToLightbox = (src: string, message: Message): NavigateAction =>
   NavigationActions.navigate({ routeName: 'lightbox', params: { src, message } });
 
-export const navigateToLoading = (): NavigateAction =>
-  NavigationActions.navigate({ routeName: 'loading' });
-
 export const navigateToLanguage = (): NavigateAction =>
   NavigationActions.navigate({ routeName: 'language' });
 
@@ -67,9 +70,6 @@ export const navigateToDiagnostics = (): NavigateAction =>
 export const navigateToWelcomeHelp = (): NavigateAction =>
   NavigationActions.navigate({ routeName: 'welcome-help' });
 
-export const navigateToWelcomeScreen = (): NavigateAction =>
-  NavigationActions.navigate({ routeName: 'welcome' });
-
 export const navigateToVariables = (): NavigateAction =>
   NavigationActions.navigate({ routeName: 'variables' });