docs: improve the description of cp (#1498)

Signed-off-by: Xiaoxuan Wang <xiaoxuanwang@microsoft.com>
Signed-off-by: Billy Zha <jinzha1@microsoft.com>

cmd/oras/root/cp.go

@@ -58,7 +58,7 @@ func copyCmd() *cobra.Command {
 		Use:     "cp [flags] <from>{:<tag>|@<digest>} <to>[:<tag>[,<tag>][...]]",
 		Aliases: []string{"copy"},
 		Short:   "Copy artifacts from one target to another",
-		Long: `Copy artifacts from one target to another
+		Long: `Copy artifacts from one target to another. When copying an image index, all of its manifests will be copied
 
 Example - Copy an artifact between registries:
   oras cp localhost:5000/net-monitor:v1 localhost:6000/net-monitor-copy:v1