better prune and system df

Signed-off-by: allencloud <allen.sun@daocloud.io>

command/system/df.go

@@ -19,7 +19,7 @@ func NewDiskUsageCommand(dockerCli *command.DockerCli) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "df [OPTIONS]",
 		Short: "Show docker disk usage",
-		Args:  cli.RequiresMaxArgs(1),
+		Args:  cli.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runDiskUsage(dockerCli, opts)
 		},

command/system/prune.go

@@ -15,13 +15,13 @@ type pruneOptions struct {
 	all   bool
 }
 
-// NewPruneCommand creates a new cobra.Command for `docker du`
+// NewPruneCommand creates a new cobra.Command for `docker prune`
 func NewPruneCommand(dockerCli *command.DockerCli) *cobra.Command {
 	var opts pruneOptions
 
 	cmd := &cobra.Command{
 		Use:   "prune [OPTIONS]",
-		Short: "Remove unused data.",
+		Short: "Remove unused data",
 		Args:  cli.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runPrune(dockerCli, opts)