trust: address review feedback, refactor to align with existing cli/command semantics

Signed-off-by: Riyaz Faizullabhoy <riyaz.faizullabhoy@docker.com>

cli/command/trust/cmd.go

@@ -10,7 +10,7 @@ import (
 func NewTrustCommand(dockerCli command.Cli) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "trust",
-		Short: "Sign images to establish trust",
+		Short: "Manage trust on Docker images",
 		Args:  cli.NoArgs,
 		RunE:  command.ShowHelp(dockerCli.Err()),
 	}