docs: improve docs about tokio::main

websockets/echo/src/main.rs

@@ -18,6 +18,7 @@ async fn echo_ws(req: HttpRequest, stream: web::Payload) -> Result<HttpResponse,
     ws::start(MyWebSocket::new(), &req, stream)
 }
 
+// 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"));