IDM_CREATE_DEMO_USERS need to be run with `ocis server` not with `ocis init`

## Describe the bug

The documentation mentions in multiple places to create demo users by running `ocis init` with `IDM_CREATE_DEMO_USERS=true` this does not work for me. I only can create demo users when starting the server (`ocis server`) with `IDM_CREATE_DEMO_USERS=true`

examples in docs:
- https://doc.owncloud.com/ocis/next/deployment/security/security.html
- https://doc.owncloud.com/ocis/next/deployment/general/general-info.html#default-users-and-groups

also creating the demo users only works on the very first start of `ocis server` if the `IDM_CREATE_DEMO_USERS` setting is not set on the first start, the demo users will not be created even the setting is set to true on later starts

## Expected behavior
not sure if this is a docs issue or an issue in ocis

## Setup
Using `ocis-3.0.0-rc.4-linux-amd64` binary