Need more documentation for "startup-order" / "rollback-order"

[thaJeztah]

Docker 17.05 added an option to start new instances of a service before killing the previous one (see moby/moby#30261, moby/moby#31955), however, documentation is very minimal on this feature.

We should add a better description, and example to show its use.

/cc @MistyHacks

[ahh-docker]

@thaJeztah Is still valid with 18.09? Thanks.

[olejorgenb]

I'd say this issue is still valid. It's not really possible to understand in details how the 'start-first' option interacts with the load-balancer for instance. You can guess it work in the sane way, but there's no way to tell from the docs unless I'm missing something.

Incorporating info from moby/moby#30321 should be done IMO.