Update node about container_manager variable (#7911)

I was deploy my cluster with separate etcd cluster and not intersect with kube_control_plane or kube_node. And I want to run etcd cluster in docker but still used containerd to make container runtime for all other nodes. Therefore, I was added note to this doc for everyone 

Thank !

docs/containerd.md

@@ -7,6 +7,8 @@ _To use the containerd container runtime set the following variables:_
 
 ## k8s_cluster.yml
 
+When kube_node_ contains etcd, you define your etcd cluster to be as well schedulable for Kubernetes workloads. Thus containerd and dockerd can not run same time, must be set to bellow for running etcd cluster with only containerd.
+
 ```yaml
 container_manager: containerd
 ```