tools/demo-server: note that aborting on unknown events is bad

Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/322>

tools/eis-demo-server.c

@@ -547,6 +547,13 @@ eis_demo_server_printf_handle_event(struct eis_demo_server *server,
 		}
 		break;
 	default:
+		/* This is a demo server and we abort to make it easy to debug
+		 * a missing implementation of some new event.
+		 *
+		 * Do not do this in a real EIS implementation, unknown
+		 * events must be passed to eis_event_unref() and otherwise
+		 * ignored.
+		 */
 		abort();
 	}
 	return 0;