Documentation for `attest --type` parameter is missing

[toddysm]

Description

cosign attest --help lists a --type parameter but it is not clear what values this parameter can accept. It seems it is not required but would be good to know what is the purpose of the parameter and what values are accepted. Adding documentation to the following page may be good https://docs.sigstore.dev/cosign/attestation

[Neilblaze]

@toddysm I'd like to take this up! Also, can you suggest any specific content(s) that should can be added for the same? Thanks! :)

[toddysm]

I don't know what is the purpose of that parameter so I cannot propose text. At a min it would be good to document the accepted values and the meaning of the parameter.