Clarify At-Most-One Semantic For DaemonSet Update (kubernetes#19283)

* Clarify At-Most-One Semantic For DaemonSet Update

Make it explicit, that at most one pod will run on each node while updating a daemonset. As mentioned here: kubernetes/kubernetes#48841

* Clarify Update Strategy For DaemonSet Pods Only

content/en/docs/tasks/manage-daemon/update-daemon-set.md

@@ -33,7 +33,7 @@ DaemonSet has two update strategy types:
 * RollingUpdate: This is the default update strategy.  
   With `RollingUpdate` update strategy, after you update a
   DaemonSet template, old DaemonSet pods will be killed, and new DaemonSet pods
-  will be created automatically, in a controlled fashion.
+  will be created automatically, in a controlled fashion. At most one pod of the DaemonSet will be running on each node during the whole update process.
 
 ## Performing a Rolling Update