autocomplete [nfc]: Add TODO comments for future autocomplete criteria

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -234,6 +234,10 @@ class MentionAutocompleteView extends ChangeNotifier {
     required String? topic,
     required PerAccountStore store,
   }) {
+    // TODO(#234): give preference to "all", "everyone" or "stream".
+
+    // TODO(#618): give preference to subscribed users first.
+
     if (streamId != null) {
       final result = compareByRecency(userA, userB,
         streamId: streamId,