nav: Fix ordering slightly.

I'd stuck this comment block just below one of the navigate-somewhere
actions and above all the others, which could be a bit confusing.
Move it up so it doesn't separate them.

src/nav/navActions.js

@@ -16,16 +16,16 @@ export const navigateBack = () => (dispatch: Dispatch, getState: GetState): Navi
   // $FlowFixMe
   dispatch(NavigationActions.pop({ n: getSameRoutesCount(getState()) }));
 
-/** Only call this via `doNarrow`.  See there for details. */
-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' });
 
+/** Only call this via `doNarrow`.  See there for details. */
+export const navigateToChat = (narrow: Narrow): NavigateAction =>
+  NavigationActions.navigate({ routeName: 'chat', params: { narrow } });
+
 export const navigateToUsersScreen = (): NavigateAction =>
   NavigationActions.navigate({ routeName: 'users' });