A recent comment on https://review.opendev.org/#/c/665835
pointed out that we should likely make some changes and a fix
a missing check for the introspection_vif_port_id which was
likely introduced after this functionality was originally
written.
Also adds some documentation on the subject since we lack
docs even pointing out how to delete a port. :\
Change-Id: I0ba8a3741eefa80eb56e25a1b339f8433b3fc0dc