Document behavior of constraints on a global mode service

[programmerq]

File: engine/reference/commandline/service_create.md, CC @mstanleyjones

The behavior seems to be fairly clear if you find the original pull request that enables the current behavior: moby/swarmkit#1570

The current way that this document page reads, it is unclear what the exact intended behavior is: global mode services do respect constraints.

[johndmulhausen]

Issue moved to docker/cli #592 via ZenHub

[docker-robott]

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