Purpose of the `HOSTNAME` argument in the `cfssl selfsign` command

Hi! 👋

What is the purpose of the `HOSTNAME` argument of the `cfssl selfsign` command?

```
~ ❯ cfssl selfsign --help
cfssl selfsign -- generate a new self-signed key and signed certificate

Usage of gencert:
cfssl selfsign HOSTNAME CSRJSON

WARNING: this should ONLY be used for testing. This should never be
used in production.

WARNING: self-signed certificates are insecure; they do not provide
the authentication required for secure systems. Use these at your own
risk.

Arguments:
HOSTNAME: Hostname for the cert
CSRJSON: JSON file containing the request, use '-' for reading JSON from stdin

Flags:
-config="": path to configuration file
-loglevel=1: Log level (0 = DEBUG, 5 = FATAL)
```

In the help of this command it appears as a mandatory argument before `CSRJSON`, but checking the created certificate I don't see that it affects it at all. I have been reviewing the source code for this particular command and I don't see anything done with the `HOSTNAME`parameter. Why is it necessary to specify it in the command?

Thank you very much!

Best regards

Start at the cfssl selfsign command entry point and trace how the HOSTNAME argument is handled during certificate generation. Compare that behavior with the generated certificate and the existing --help text; done means documenting the argument's purpose or correcting the usage text if it is unused.

Written by the indexing model from the issue text.