Update documentation based on RHODS-2442.

This changes the default groups mapped to RHODS users & admins in the config map - see the config map sections in the description of RHODS-2442.

This change means that OSD users will automatically have access & OSD admins will be RHODS admins if no changes are made. We should document that part, and the fact that this is the default behavior as of 1.8. We need to be explicit that the original default behaviour was not the same, as existing cluster behavior won't change and that might confuse admins later.

If admins of new clusters want to restrict access, they can follow the same process currently documented - i.e. change the config map to map to RHODS users / admins or other desired groups. So we're not changing the ability to restrict access or the methods, just the default.