User Info Fetcher: Document data differences between backends, and/or how to query it manually

[nightkr]

Currently, we only document the Keycloak "format" (username is the keycloak username, groups are "file-system-style paths"), but not the AD one (username is the Kerberos UPN, groups are distinguished names), or how it might differ between them (beyond a slight mention that there is a difference).

We should give examples for each, and/or give an easy example for how to query it yourself to see the data format that applies to your environment.