document --secret/--private option in `key remove`

[preminger]

Which Document page:

https://docs.sylabs.io/guides/latest/user-guide/key_commands.html

Expected results:

Following sylabs/singularity#1415, the documentation for the key remove subcommand should discuss the --secret/--private option.

Also, since --private as well as the shorthand -s are now available as synonyms for --secret in key import and key export as well, the documentation should reflect these synonyms throughout.

Actual results:

Not documented yet.