Document `openstack complete` help and usage
Document `openstack complete` help and usage

Currently we have no documentation for the `complete` command.

Change-Id: I9faf38341bb57a61869ddbef406a022219a7c62c
Closes-Bug: 1623860

openstack/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient