Add details about rolling restarts behaviors (elastic#7372)

docs/operating-eck/upgrading-eck.asciidoc

@@ -99,6 +99,8 @@ Upgrading the operator results in a one-time update to existing managed resource
 
  1.6, 1.9, 2.0, 2.1, 2.2, 2.4, 2.5, 2.6, 2.7, 2.8
 
+NOTE: Stepping over one of these versions, for example, upgrading ECK from 2.6 to 2.9, still triggers a rolling restart.
+
 If you have a very large Elasticsearch cluster or multiple Elastic Stack deployments, this rolling restart might be disruptive or inconvenient. To have more control over when the pods belonging to a particular deployment should be restarted, you can <<{p}-exclude-resource,add an annotation>> to the corresponding resources to temporarily exclude them from being managed by the operator. When the time is convenient, you can remove the annotation and let the rolling restart go through.
 
 CAUTION: Once a resource is excluded from being managed by ECK, you will not be able to add/remove nodes, upgrade Stack version, or perform other <<{p}-orchestrating-elastic-stack-applications, orchestration tasks>> by updating the resource manifest. You must remember to remove the exclusion to ensure that your Elastic Stack deployment is continually monitored and managed by the operator.