Docs missing for node platform constraints

[estesp]

As of the latest published docs (17.09), the node constraints for platform that were added in moby/swarmkit#1678 are not documented in the list of constraints shown on the docker service create reference documentation

I can attempt to add a PR for this, but wanted to at least get an issue open for now in case anyone else has more time to attack this.

[thaJeztah]

I assume this affects both the CLI reference documentation, and the API docs, correct?

[estesp]

that's true; it would affect the API for service create and update as well.