Reworded nargs='?' explanation for better clarity
Reworded nargs='?' explanation for better clarity

Change-Id: Iae2c71695ed18504690cc953b4128aeaf3196275

Juniper/python-neutronclient,Juniper/python-neutronclient,eayunstack/python-neutronclient,openstack/python-neutronclient,eayunstack/python-neutronclient,huntxu/python-neutronclient,huntxu/python-neutronclient,openstack/python-neutronclient,rackerlabs/rackspace-python-neutronclient,noironetworks/python-neutronclient,noironetworks/python-neutronclient,rackerlabs/rackspace-python-neutronclient