It is actually mentioned in [gpg man page][1] 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.

 ...
```


 [1]: https://www.gnupg.org/documentation/manuals/gnupg24/gpg.1.html