[Documentation]: USERS_FILE is compatible with multiple auth backends

[Matt]

The documentation specifically lists the following under the 'Google authentication user restrictions` heading:

export USERS_FILE='/etc/confidant/users.yaml'
export USER_EMAIL_SUFFIX='@example.com'

This however is compatible for use, at the very least, with SAML, and possibly other providers. It would be beneficial if this were to be explained or put into a more general section. Additionally, it would be clearer if there was an example of the format of the file, i.e.:

users.yaml:
- [e-mail]
- [e-mail]