Comment 7 for bug 1638714

Another bit for 2.4+ - we need to be a bit clearer about what we are actually upgrading. We want to avoid user confusion around upgrading model vs upgrading the controller. As part of this exercise, I've added an alias for 'upgrade-juju' - 'upgrade-model' since this is what we are actually doing here. PR: https://github.com/juju/juju/pull/8440