Add --force to node removal

Signed-off-by: Diogo Monica <diogo.monica@gmail.com>
(cherry picked from commit a327c231b5)
Signed-off-by: Tibor Vass <tibor@docker.com>

docs/reference/commandline/node_rm.md

@@ -13,7 +13,7 @@ parent = "smn_cli"
 # node rm
 
 ```markdown
-Usage:  docker node rm NODE [NODE...]
+Usage:  docker node rm [OPTIONS] NODE [NODE...]
 
 Remove one or more nodes from the swarm
 
@@ -21,6 +21,7 @@ Aliases:
   rm, remove
 
 Options:
+      --force  Force remove an active node
       --help   Print usage
 ```
 
@@ -32,6 +33,24 @@ Example output:
     $ docker node rm swarm-node-02
     Node swarm-node-02 removed from swarm
 
+Removes nodes from the swarm that are in the down state. Attempting to remove
+an active node will result in an error:
+
+```bash
+$ docker node rm swarm-node-03
+Error response from daemon: rpc error: code = 9 desc = node swarm-node-03 is not down and can't be removed
+```
+
+If a worker node becomes compromised, exhibits unexpected or unwanted behavior, or if you lose access to it so
+that a clean shutdown is impossible, you can use the force option.
+
+```bash
+$ docker node rm --force swarm-node-03
+Node swarm-node-03 removed from swarm
+```
+
+Note that manager nodes have to be demoted to worker nodes before they can be removed
+from the cluster.
 
 ## Related information