WebSocket.addError needs documentation

[ltOgt]

Related to #30920 and #31504.

When working with simple streams:

import 'dart:async';

main() {
  StreamController c = StreamController();
  c.stream.listen(
    (event) {
      print("[<<<]$event[>>>]");
    },
    onError: (error) {
      print("[<<<]$error[>>>]");
    },
    onDone: () => print("done"),
    cancelOnError: false,
  );
  c.add("OK");
  c.addError("error");
}

Handles both add and addError as expected.

When working with websockets:
(client.dart)

import 'dart:io';

main() async {
  late WebSocket socket;
  socket = await WebSocket.connect(
    'ws://localhost:4040/test',
  );

  socket.listen(
    (event) {
      print("[<<<]$event[>>>]");
    },
    onError: (error) {
      print("[<<<]$error[>>>]");
    },
    onDone: () => print("done"),
    cancelOnError: false,
  );
}

(server.dart)

import 'dart:io';

main() async {
  HttpServer server = await HttpServer.bind('127.0.0.1', 4040);

  await for (HttpRequest req in server) {
    if (req.uri.path == '/test') {
      // Upgrade a HttpRequest to a WebSocket connection.
      WebSocket socket = await WebSocketTransformer.upgrade(req);

      socket.add("OK");
      socket.addError("error");
    }
  }
}

server.dart will throw

Unhandled exception:
error

As I understood it, the exception is thrown because no listener for onError exists, since only the client listens for the data.

Looking back it makes sense that the socket can only send one stream of data, but the API was somewhat confusing.
It led me to believe that I can simply approach the problem like I am used to with simple streams.