Add documentation for setup ceph client authentication

There is an option [cluster_client_name](https://github.com/ceph/ceph-iscsi/blob/89964fd399c62230d8fb41124c960db701280add/iscsi-gateway.cfg_sample#L10C3-L10C22) in the `iscsi-gateway.cfg` config. I don't have the opportunity to use the `client.admin` user. Could you describe precisley in the documentation the minimal set of [capabilities](https://docs.ceph.com/en/reef/rados/operations/user-management/#authorization-capabilities) required for work ceph-iscsi without access denied. For example like [this](https://docs.ceph.com/en/reef/rbd/rbd-openstack/#setup-ceph-client-authentication).
I did it like this:

` ceph auth get-or-create client.iscsi mon 'profile rbd, allow command "osd blacklist", allow command "osd blocklist"' osd 'profile rbd' `

Start with the cluster_client_name option in iscsi-gateway.cfg_sample and compare the requested setup with the linked Ceph authorization capabilities documentation and RBD authentication example. Done means the ceph-iscsi documentation clearly states the minimal client capabilities and explains how to configure a non-client.admin user without access-denied errors.

Written by the indexing model from the issue text.