[help text]: clarify --password-file when passed to step ca certificate

[tashian]

Update the help on --password-file when used with step ca certificate. The current text is confusing. Right now it says The path to the file containing the password to encrypt or decrypt the private key., but --password-file is actually an alias for --provisioner-password-file.

Two options:

• It could be grouped under --provisioner-password-file as an alias
• Or, since --password-file is for historical reasons, we may want to hide it from step ca provisioner --help.