Add documentation to Navigator (#31851)

packages/flutter/lib/src/widgets/navigator.dart

@@ -394,6 +394,16 @@ class NavigatorObserver {
 /// manages a stack of [Route] objects and provides methods for managing
 /// the stack, like [Navigator.push] and [Navigator.pop].
 ///
+/// When your user interface fits this paradigm of a stack, where the user
+/// should be able to _navigate_ back to an earlier element in the stack,
+/// the use of routes and the Navigator is appropriate. On certain platforms,
+/// such as Android, the system UI will provide a back button (outside the
+/// bounds of your application) that will allow the user to navigate back
+/// to earlier routes in your application's stack. On platforms that don't
+/// have this build-in navigation mechanism, the use of an [AppBar] (typically
+/// used in the [Scaffold.appBar] property) can automatically add a back
+/// button for user navigation.
+///
 /// ### Displaying a full-screen route
 ///
 /// Although you can create a navigator directly, it's most common to use