cmd/docker: enable cobra completion descriptions

This patch enables descriptions on the CLI completion script.
It used to be disabled due to the CLI historically not supporting
cobra v2 completions, as seen by this patch
https://github.com/docker/cli/commit/cbec75e2f3f5afd3334db693829a6b150612076c.

As an escape hatch, the user can set the `DOCKER_CLI_DISABLE_COMPLETION_DESCRIPTION`
environment variable to disable the completion description when
generating the completion file with `docker completion <fish|bash|zsh>`.

Signed-off-by: Alano Terblanche <18033717+Benehiko@users.noreply.github.com>

cli-plugins/plugin/plugin.go

@@ -158,7 +158,7 @@ func newPluginCommand(dockerCli *command.DockerCli, plugin *cobra.Command, meta
 		CompletionOptions: cobra.CompletionOptions{
 			DisableDefaultCmd:   false,
 			HiddenDefaultCmd:    true,
-			DisableDescriptions: true,
+			DisableDescriptions: os.Getenv("DOCKER_CLI_DISABLE_COMPLETION_DESCRIPTION") != "",
 		},
 	}
 	opts, _ := cli.SetupPluginRootCommand(cmd)