Followup to #2456: update examples, add trust

- belatedly incorporate review feedback from baude
- add usage synopsis for trust-set and trust-show

Signed-off-by: Ed Santiago <santiago@redhat.com>

cmd/podman/exists.go

@@ -40,7 +40,7 @@ var (
 			imageExistsCommand.GlobalFlags = MainGlobalOpts
 			return imageExistsCmd(&imageExistsCommand)
 		},
-		Example: `podman image exists imageID`,
+		Example: `podman image exists alpine || podman pull alpine`,
 	}
 
 	_containerExistsCommand = &cobra.Command{
@@ -53,7 +53,7 @@ var (
 			return containerExistsCmd(&containerExistsCommand)
 
 		},
-		Example: `podman container exists containerID`,
+		Example: `podman container exists myctr || podman run --name myctr [etc...]`,
 	}
 
 	_podExistsCommand = &cobra.Command{
@@ -65,7 +65,7 @@ var (
 			podExistsCommand.GlobalFlags = MainGlobalOpts
 			return podExistsCmd(&podExistsCommand)
 		},
-		Example: `podman pod exists podID`,
+		Example: `podman pod exists mypod || podman pod create --name mypod`,
 	}
 )
 

cmd/podman/trust_set_show.go

@@ -23,7 +23,7 @@ var (
 	showTrustCommand    cliconfig.ShowTrustValues
 	setTrustDescription = "Set default trust policy or add a new trust policy for a registry"
 	_setTrustCommand    = &cobra.Command{
-		Use:     "set",
+		Use:     "set [flags] REGISTRY",
 		Short:   "Set default trust policy or a new trust policy for a registry",
 		Long:    setTrustDescription,
 		Example: "",
@@ -36,7 +36,7 @@ var (
 
 	showTrustDescription = "Display trust policy for the system"
 	_showTrustCommand    = &cobra.Command{
-		Use:   "show",
+		Use:   "show [flags] [REGISTRY]",
 		Short: "Display trust policy for the system",
 		Long:  showTrustDescription,
 		RunE: func(cmd *cobra.Command, args []string) error {