[docs] missing placement constraints grammar

[alexanderkjeldaas]

Description

The placement constraints grammar is not documented.

Steps to reproduce the issue:

1. docker help create
2. search for "placement constraints" on docker.com

Describe the results you received:

No documentation.

Describe the results you expected:

Documentation of the grammar, such as how not equal is written, whether and and or can be used, and what the grammar is etc.

[thaJeztah]

The help output does not give full documentation, but you can find documentation on the constraints here; https://github.com/docker/docker/blob/v17.03.0-ce/docs/reference/commandline/service_create.md#specify-service-constraints---constraint

That part of the docs is currently not published on docs.docker.com due to some changes in tooling, but will soon be available here; https://docs.docker.com/engine/reference/commandline/service_create/

Note that docker service create is not the same as docker create; the latter one creates a container, not a service, and does not have placement constraints