Show error / update config & examples, for multiple ws accounts

[tiagosiebler]

"Websocket no longer supported multiple account batch login."
https://www.okx.com/docs-v5/log_en/#2023-04-19

Instead, the expectation is that you create one connection per account. With the SDK, you can do this by making one WebsocketClient instance per account. I'll look to update the parameter to make that clearer in future...

Need to:

• update docs to remove suggestions multiple accounts are supported in one wsclient instance
• update examples to ^
• throw hard error if multiple accounts are provided in ws config?