docs: improve docs about tokio::main

websockets/chat/src/main.rs

@@ -45,6 +45,7 @@ async fn get_count(count: web::Data<AtomicUsize>) -> impl Responder {
     format!("Visitors: {current_count}")
 }
 
+// the actor-based WebSocket examples REQUIRE `actix_web::main` for actor support
 #[actix_web::main]
 async fn main() -> std::io::Result<()> {
     env_logger::init_from_env(env_logger::Env::new().default_filter_or("info"));