Clarify node affinity in ScheduleDaemonSetPods

> If node affinity of the DaemonSet pod already exists, it is replaced.

The original statement made it sound like the original node affinity was ignored when selecting the target node. Added clarification it is not the case. 

For reference, the DaemonSet controller behavior. 
https://github.com/kubernetes/kubernetes/blob/master/pkg/controller/daemon/daemon_controller.go#L1194

Author: jqmichael

content/en/docs/concepts/workloads/controllers/daemonset.md

@@ -125,7 +125,7 @@ That introduces the following issues:
 scheduler instead of the DaemonSet controller, by adding the `NodeAffinity` term
 to the DaemonSet pods, instead of the `.spec.nodeName` term. The default
 scheduler is then used to bind the pod to the target host. If node affinity of
-the DaemonSet pod already exists, it is replaced. The DaemonSet controller only
+the DaemonSet pod already exists, it is replaced (the original node affinity was taken into account before selecting the target host). The DaemonSet controller only
 performs these operations when creating or modifying DaemonSet pods, and no
 changes are made to the `spec.template` of the DaemonSet.