Docs: describe the configured ELB health check

[molexx]

Expected behavior

Docs 'Load balancer' page contains a small section on the health check that has been set up.

Actual behavior

Confusion as to why there's a health check set up targetting port 44554, yet when sshing to the servers 'docker ps' shows nothing listening on that port.

Information

Comments in #52 explain what's going on:

BTW 44554 is our diagnostic server, it runs directly on the host. If it stops responding the host would reboot. So it might be an issue with that server.

Originally posted by @kencochrane in https://github.com/docker/for-aws/issues/52#issuecomment-310754723

The diagnostic server doesn't run as a container, it runs directly on the host, and it monitors docker and other items. Because of this, it isn't visible from docker ps.

Originally posted by @kencochrane in https://github.com/docker/for-aws/issues/52#issuecomment-310769922