doc: clarify socket and keepAliveTimeout interaction

Socket timeouts set using the connection event are replaced by
server.keepAliveTimeout when a response is handled.

Socket timeouts set in the connection event are replaced after a response is sent in the resOnFinish handler

server.on('connection', function(socket) {
  socket.setTimeout(60000);
});

In this case, the socket timeout is 60 seconds until a response is sent, at which point it becomes 5 seconds (the server.keepAliveTimeout default). This bit me when working with load balancers that maintain long-lived connections.

I have added a note to the connection event docs to the effect that is preferred to use server.keepAliveTimeout.

Checklist

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