Docs: add note about management commands targeting a manager

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/node_inspect.md

@@ -34,6 +34,10 @@ given template for each result. Go's
 [text/template](http://golang.org/pkg/text/template/) package describes all the
 details of the format.
 
+> **Note**: This is a cluster management command, and must be executed on a swarm
+> manager node. To learn about managers and workers, refer to the [Swarm mode
+> section](https://docs.docker.com/engine/swarm/) in the documentation.
+
 ## Examples
 
 ### Inspect a node