It is actually mentioned in gpg man page now in --list-options - show-usage option description.
--list-options parameters This is a space or comma delimited string that gives options used when listing keys and signatures (that is, --list-keys, --check-signatures, --list-public-keys, --list-secret-keys, and the --edit-key functions). Options can be prepended with a no- (after the two dashes) to give the opposite meaning. The op‐ tions are: ... show-usage Show usage information for keys and subkeys in the stan‐ dard key listing. This is a list of letters indicating the allowed usage for a key (E=encryption, S=signing, C=certification, A=authentication). Defaults to yes. ...