Configuration: Document `irods_authentication_scheme` in `irods_environment.json`

[alanking]

There is an example irods_environment.json documented here with explanations for all the different keys that can exist therein: https://docs.irods.org/5.0.1/system_overview/configuration/#irodsirods_environmentjson It does not have an irods_authentication_scheme, which is required for non-native authentication. We should document that.

For reference, irods_authentication_scheme is documented elsewhere: https://docs.irods.org/5.0.1/plugins/pluggable_authentication/#user-setup