docs(client): add more clarity to config:set
docs(client): add more clarity to config:set

You can specify multiple environment variables to in config:set. This
change is completely backwards-compatible. As an example, this is the
output with just a print statement in the body:

    ><> deis config:set A=b C=d E=f
    {'values': '{"A": "b", "C": "d", "E": "f"}'}

deis/workflow,deis/workflow,deis/workflow