Clarify the DRAIN state w/ regard to containers (docker#5811)

* Clarify the DRAIN state w/ regard to containers

Bare containers are not state-reconciled, and are not affected by the swarm DRAIN state

Author: adamancini

engine/swarm/swarm-tutorial/drain-node.md

@@ -14,6 +14,12 @@ availability. `DRAIN` availability  prevents a node from receiving new tasks
 from the swarm manager. It also means the manager stops tasks running on the
 node and launches replica tasks on a node with `ACTIVE` availability.
 
+> **Important**: Setting a node to `DRAIN` does not remove standalone containers from that node,
+> such as those created with `docker run`, `docker-compose up`, or the Docker Engine
+> API. A node's status, including `DRAIN`, only affects the node's ability to schedule
+> swarm service workloads.
+{:.important}
+
 1.  If you haven't already, open a terminal and ssh into the machine where you
     run your manager node. For example, the tutorial uses a machine named
     `manager1`.