action_sheet [nfc]: Comment on user-topic button about client-side check

Zixuan points out that this could be helpful here:
  zulip#1301 (comment)

This probably makes sense to not do until zulip#1078, when we'll have
separate buttons for each of the four possible user-topic states.

This same comment *could* be added on various other action-sheet
buttons. But realistically it wouldn't make a difference on any that
we've implemented so far: all the others (besides this and
ResolveUnresolveButton) are on the message action sheet, and they're
all about actions that would only be done by the self-user --
starring, marking as unread, etc. -- so they can't be done without
the user's awareness, and will rarely be done anyway because you'd
need to do it on a different client.

lib/widgets/action_sheet.dart

@@ -361,6 +361,10 @@ class UserTopicUpdateButton extends ActionSheetMenuItemButton {
   }
 
   @override void onPressed() async {
+    // Could implement client-side check on whether the action was done between
+    // opening the action sheet and pressing the button.
+    // Like in ResolveUnresolveButton;
+    // see also dartdoc of [ActionSheetMenuItemButton].
     try {
       await updateUserTopicCompat(
         PerAccountStoreWidget.of(pageContext).connection,