Updated package-import help description
Updated package-import help description

Improved category metavar.
Removed incorrect note of possibility to use comma in categorie list
Added note about possibility to use -- to separate positional FILE
arguments from the rest of arguments.

Change-Id: Ia6e8f7c35bfb912d526db23a6bf8e766e7765187
Closes-Bug: #1478912

openstack/python-muranoclient,openstack/python-muranoclient