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
I'm confused by the second sentence. Would this say the same thing?

> Deis may rely on particular versions of etcd, fleet, and docker, and its customized user-data file may make assumptions about the CoreOS release.