Fix bug in Autocomplete example (#127219)

This example was incorrectly throwing away results from a query when multiple queries were pending at once.   Thanks to @sun-jiao in https://github.com/flutter/flutter/pull/127019#issuecomment-1552347037 for pointing this out.

I also added a quick  `Text` widget explaining what to do to use the examples.  Since there are only three small possible `options`, it's easy to type into the field and not get any results and wonder what's wrong.
diff --git a/examples/api/lib/material/autocomplete/autocomplete.1.dart b/examples/api/lib/material/autocomplete/autocomplete.1.dart
index 7e3798e..abee231 100644
--- a/examples/api/lib/material/autocomplete/autocomplete.1.dart
+++ b/examples/api/lib/material/autocomplete/autocomplete.1.dart
@@ -18,8 +18,14 @@
         appBar: AppBar(
           title: const Text('Autocomplete Basic User'),
         ),
-        body: const Center(
-          child: AutocompleteBasicUserExample(),
+        body: Center(
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: <Widget>[
+              Text('Type below to autocomplete the following possible results: ${AutocompleteBasicUserExample._userOptions}.'),
+              const AutocompleteBasicUserExample(),
+            ],
+          ),
         ),
       ),
     );