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 also didn't understand what was meant by "Deis's customized cloud-config file
sometimes makes assumptions about what version of Deis it's running on."