Add comment explaining network-create flow for Swarm

Signed-off-by: Rob Murray <rob.murray@docker.com>

Author: robmry

api/server/router/network/network_routes.go

@@ -213,6 +213,10 @@ func (n *networkRouter) postNetworkCreate(ctx context.Context, w http.ResponseWr
 		return libnetwork.NetworkNameError(create.Name)
 	}
 
+	// For a Swarm-scoped network, this call to backend.CreateNetwork is used to
+	// validate the configuration. The network will not be created but, if the
+	// configuration is valid, ManagerRedirectError will be returned and handled
+	// below.
 	nw, err := n.backend.CreateNetwork(create)
 	if err != nil {
 		if _, ok := err.(libnetwork.ManagerRedirectError); !ok {