Change the docker-tag usage text to be clearer

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

cli/command/image/tag.go

@@ -18,8 +18,8 @@ func NewTagCommand(dockerCli *command.DockerCli) *cobra.Command {
 	var opts tagOptions
 
 	cmd := &cobra.Command{
-		Use:   "tag IMAGE[:TAG] IMAGE[:TAG]",
-		Short: "Tag an image into a repository",
+		Use:   "tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]",
+		Short: "Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE",
 		Args:  cli.ExactArgs(2),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			opts.image = args[0]

docs/reference/commandline/tag.md

@@ -16,9 +16,9 @@ keywords: "tag, name, image"
 # tag
 
 ```markdown
-Usage:  docker tag IMAGE[:TAG] IMAGE[:TAG]
+Usage:  docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG]
 
-Tag an image into a repository
+Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE
 
 Options:
       --help   Print usage

man/docker-tag.1.md

@@ -2,12 +2,12 @@
 % Docker Community
 % JUNE 2014
 # NAME
-docker-tag - Tag an image into a repository
+docker-tag - Create a tag `TARGET_IMAGE` that refers to `SOURCE_IMAGE`
 
 # SYNOPSIS
 **docker tag**
 [**--help**]
-NAME[:TAG] NAME[:TAG]
+SOURCE_NAME[:TAG] TARGET_NAME[:TAG]
 
 # DESCRIPTION
 Assigns a new alias to an image in a registry. An alias refers to the