code: public List<RequestFuture<?>> broadcast(String content)
      {
          Checks.notBlank(content, "Content");
          Checks.check(content.length() <= 2000, "Content may not exceed 2000 characters!");
          final RequestBody body = WebhookClient.newBody(new JSONObject().put("content", content).toString());
          final List<RequestFuture<?>> callbacks = new ArrayList<>(webhooks.size());
          for (WebhookClient webhook : webhooks)
              callbacks.add(webhook.execute(body));
          return callbacks;
      }
docs: Sends the provided text message
  to all registered {@link net.dv8tion.jda.webhook.WebhookClient WebhookClients}.
  
  @param  content
  The text that should be sent to the clients
  
  @throws java.lang.IllegalArgumentException
  If the provided content is {@code null} or blank
  @throws java.util.concurrent.RejectedExecutionException
  If any of the receivers has been shutdown
  
  @return A list of {@link java.util.concurrent.Future Future} instances
  representing all message tasks.