docs(managing_deis): add warning to CoreOS upgrade

@danielfarrell mentioned in IRC today that he followed our instructions to upgrade CoreOS, and ran into the insecure registry issue we're addressing in #2579. He pointed out that we have no warnings of any kind around the upgrading CoreOS documentation, which is potentially destructive (such as this case, where we haven't yet caught up to CoreOS's changes). This PR fleshes out the issue of upgrading CoreOS and adds an appropriate warning.

closes #2581
Mainly, I meant that our customizations there are the most likely to be affected by changes to CoreOS. We expect specific services to exist, etc.

Additionally, new cloud-config and upgrades to CoreOS need to be handled in tandem - typically, one cannot upgrade one without the other.