Boot options: add help text to state selection

To indicate this is a menu you can enter to select a previous
state to boot. Also rename "Latest version" to "Latest state" as
that is more correct.
Thanks waddlesplash for pointers.

Fixes #14362

src/system/boot/loader/menu.cpp

@@ -1211,6 +1211,8 @@ add_boot_volume_item(Menu* menu, Directory* volume, const char* name)
 	}
 
 	if (subMenu != NULL && subMenu->CountItems() > 1) {
+		item->SetHelpText(
+			"Enter to choose a different state to boot");
 		item->SetSubmenu(subMenu);
 	} else {
 		delete subMenu;

src/system/boot/loader/package_support.cpp

@@ -104,7 +104,7 @@ PackageVolumeState::Unset()
 const char*
 PackageVolumeState::DisplayName() const
 {
-	return fDisplayName != NULL ? fDisplayName : "Latest version";
+	return fDisplayName != NULL ? fDisplayName : "Latest state";
 }