docs(managing_deis): add warning to CoreOS upgrade

Author: carmstrong

docs/managing_deis/upgrading-deis.rst

@@ -149,9 +149,22 @@ Once all applications have been validated, the old cluster can be retired.
 Upgrading CoreOS
 ----------------
 
-Sometimes you may need to update CoreOS manually in order to get Deis to work. For example, Deis
-requires a minimum of CoreOS v472.0.0. To update CoreOS, run the following
-commands:
+By default, Deis disables CoreOS automatic updates. This is partially because of problems we've seen
+with etcd/fleet version incompatibilities as hosts in the cluster are upgraded one-by-one.
+Additionally, because Deis customizes the CoreOS cloud-config file, upgrading the CoreOS host to
+a new version without accounting for changes in the cloud-config file could cause Deis to stop
+functioning properly.
+
+While not recommended, it is possible to trigger an update of a CoreOS machine.
+
+.. important::
+
+  Enabling updates for CoreOS will result in the machine upgrading to the latest CoreOS release
+  available in a particular channel. Sometimes, new CoreOS releases make changes that will break
+  Deis. It is always recommended to provision a Deis release with the CoreOS version specified
+  in that release's provision scripts or documentation.
+
+To update CoreOS, run the following commands:
 
 .. code-block:: console