Improve docs for step ca root

command/ca/root.go

@@ -20,7 +20,33 @@ func rootComand() cli.Command {
 		UsageText: `**step ca root** <root-file>
 		[--fingerprint=<fingerprint>] [**--ca-url**=<uri>]`,
 		Description: `**step ca root** downloads and validates the root certificate from the
-certificate authority.`,
+certificate authority.
+
+## POSITIONAL ARGUMENTS
+
+<root-file>
+:  File to write root certificate (PEM format)
+
+## EXAMPLES
+
+Get the root fingerprint in the CA:
+'''
+$ step certificate fingerprint /path/to/root_ca.crt
+0d7d3834cf187726cf331c40a31aa7ef6b29ba4df601416c9788f6ee01058cf3
+'''
+
+Download the root certificate from the configured certificate authority:
+'''
+$ step ca root root_ca.crt \
+  --fingerprint 0d7d3834cf187726cf331c40a31aa7ef6b29ba4df601416c9788f6ee01058cf3
+'''
+
+Download the root certificate using a given certificate authority:
+'''
+$ step ca root root_ca.crt \
+  --ca-url https://ca.smallstep.com:9000 \
+  --fingerprint 0d7d3834cf187726cf331c40a31aa7ef6b29ba4df601416c9788f6ee01058cf3
+'''`,
 		Flags: []cli.Flag{
 			cli.StringFlag{
 				Name:  "fingerprint",
@@ -56,6 +82,7 @@ func rootAction(ctx *cli.Context) error {
 		return err
 	}
 
+	// Root already validates the certificate
 	resp, err := client.Root(fingerprint)
 	if err != nil {
 		return errors.Wrap(err, "error downloading root certificate")