Add note about version 2.5 when listing servers using --ip6
Add note about version 2.5 when listing servers using --ip6

The --ip6 filter when listing servers as a non-admin user
only applies when also using --os-compute-api-microversion
2.5 or greater. This change simply adds a note about that in
the --ip6 option help text.

We could probably get more sophisticated by trying to determine
if the user has the admin role or not and if not, and using --ip6
without microversion >= 2.5, we could error out, but that seems
excessive at this point.

Change-Id: I665c64e0bdac04c695fa119a479df43f70b0fa62
Story: #2002184
Task: #20057

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