doc/iscsi: Add description about iSCSI login redirection

  return back; after all the commands return back, the LUN will be deleted.

@sa spdk_nvme_probe

# iSCSI Login Redirection {#iscsi_login_redirection}

The SPDK iSCSI target application supports iSCSI login redirection feature.

A portal refers to an IP address and TCP port number pair, and a portal group

contains a set of portals. Users for the SPDK iSCSI target application configure

portals through portal groups.

To support login redirection feature, we utilize two types of portal groups,

public portal group and private portal group.

The SPDK iSCSI target application usually has a discovery portal. The discovery

portal is connected by an initiator to get a list of targets, as well as the list

of portals on which these target may be accessed, by a discovery session.

Public portal groups have their portals returned by a discovery session. Private

portal groups do not have their portals returned by a discovery session. A public

portal group may optionally have a redirect portal for non-discovery logins for

each associated target. This redirect portal must be from a private portal group.

Initiators configure portals in public portal groups as target portals. When an

initator logs in to a target through a portal in an associated public portal group,

the target sends a temporary redirection response with a redirect portal. Then the

initiator logs in to the target again through the redirect portal.

Users set a portal group to public or private at creation using the

`iscsi_create_portal_group` RPC, associate portal groups with a target using the

`iscsi_create_target_node` RPC or the `iscsi_target_node_add_pg_ig_maps` RPC, and

specify a up-to-date redirect portal in a public portal group for a target using

`iscsi_target_node_set_redirect` RPC.

Typically users will use the login redirection feature in scale out iSCSI target

system, which runs multiple SPDK iSCSI target applications.