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
Not really - it's not that we're relying on specific versions of etcd and fleet, it's that we've seen issues before where upgrading one host to a newer etcd/fleet would cause the cluster to lose quorum. On a second read, "in pieces" isn't really clear.