Document autolock/unlock/unlock-key

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/reference/commandline/swarm_unlock.md

@@ -0,0 +1,41 @@
+---
+title: "swarm unlock"
+description: "The swarm unlock command description and usage"
+keywords: "swarm, unlock"
+---
+
+<!-- This file is maintained within the docker/docker Github
+     repository at https://github.com/docker/docker/. Make all
+     pull requests against that repo. If you see this file in
+     another repository, consider it read-only there, as it will
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+-->
+
+# swarm unlock
+
+```markdown
+Usage:	docker swarm unlock
+
+Unlock swarm
+
+Options:
+      --help   Print usage
+```
+
+Unlocks a locked manager using a user-supplied unlock key. This command must be
+used to reactivate a manager after its Docker daemon restarts if the autolock
+setting is turned on. The unlock key is printed at the time when autolock is
+enabled, and is also available from the `docker swarm unlock-key` command.
+
+
+```bash
+$ docker swarm unlock
+Please enter unlock key:
+```
+
+## Related information
+
+* [swarm init](swarm_init.md)
+* [swarm update](swarm_update.md)