doc: clarify IncomingMessage.destroy() description

State that the 'error' event is emitted on the underlying socket, not the IncomingMessage object.

Here are the relevant parts in the code:

• IncomingMessage.destroy
• which calls Steam.destroy

Here's a program that demonstrates this

http.createServer(req=>{
  req.on('error',console.error);
  req.destroy(new Error('hi'));
}).listen(8080)

Then, curl localhost:8080 The Node.js process doesn't print the error

Checklist

• documentation is changed or added
• commit message follows commit guidelines