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"}'}

Author: Matthew Fisher

client/deis.py

@@ -1099,7 +1099,7 @@ def config_set(self, args):
         """
         Sets environment variables for an application.
 
-        Usage: deis config:set <var>=<value>... [options]
+        Usage: deis config:set <var>=<value> [<var>=<value>...] [options]
 
         Arguments:
           <var>