docs: update 'uc service rm' description that volumes are preserved

cmd/uncloud/service/rm.go

@@ -19,7 +19,12 @@ func NewRmCommand() *cobra.Command {
 		Use:     "rm SERVICE [SERVICE...]",
 		Aliases: []string{"remove", "delete"},
 		Short:   "Remove one or more services.",
-		Args:    cobra.MinimumNArgs(1),
+		Long: `Remove one or more services.
+
+The volumes used by the services are preserved and should be removed separately
+with 'uc volume rm'. Anonymous Docker volumes (automatically created from VOLUME
+directives in image Dockerfiles) are automatically removed with their containers.`,
+		Args: cobra.MinimumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			uncli := cmd.Context().Value("cli").(*cli.CLI)
 			opts.services = args