Add a note saying that we only create a single Client. (#1234)

pkgs/flutter_http_example/lib/main.dart

@@ -15,6 +15,14 @@ import 'http_client_factory.dart'
 
 void main() {
  runApp(Provider<Client>(
+ // `Provider` calls its `create` argument once when a `Client` is
+ // first requested (through `BuildContext.read<Client>()`) and uses that
+ // same instance for all future requests.
+ //
+ // Reusing the same `Client` may:
+ // - reduce memory usage
+ // - allow caching of fetched URLs
+ // - allow connections to be persisted
  create: (_) => http_factory.httpClient(),
  child: const BookSearchApp(),
  dispose: (_, client) => client.close()));