scsi: target: add driver-api document

Add a driver-api document for target/iSCSI interfaces.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
To: "Nicholas A. Bellinger" <nab@linux-iscsi.org>
Cc: linux-scsi@vger.kernel.org
Cc: target-devel@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: "James E.J. Bottomley" <jejb@linux.vnet.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Documentation/driver-api/index.rst

@@ -34,6 +34,7 @@ available subsections can be seen below.
    edac
    scsi
    libata
+   target
    mtdnand
    miscellaneous
    w1

Documentation/driver-api/scsi.rst

@@ -334,5 +334,5 @@ todo
 ~~~~
 
 Parallel (fast/wide/ultra) SCSI, USB, SATA, SAS, Fibre Channel,
-FireWire, ATAPI devices, Infiniband, I2O, iSCSI, Parallel ports,
+FireWire, ATAPI devices, Infiniband, I2O, Parallel ports,
 netlink...

Documentation/driver-api/target.rst

+=================================
+target and iSCSI Interfaces Guide
+=================================
+
+Introduction and Overview
+=========================
+
+TBD
+
+Target core device interfaces
+=============================
+
+.. kernel-doc:: drivers/target/target_core_device.c
+    :export:
+
+Target core transport interfaces
+================================
+
+.. kernel-doc:: drivers/target/target_core_transport.c
+    :export:
+
+Target-supported userspace I/O
+==============================
+
+.. kernel-doc:: drivers/target/target_core_user.c
+    :doc: Userspace I/O
+
+.. kernel-doc:: include/uapi/linux/target_core_user.h
+    :doc: Ring Design
+
+iSCSI helper functions
+======================
+
+.. kernel-doc:: drivers/scsi/libiscsi.c
+   :export:
+
+
+iSCSI boot information
+======================
+
+.. kernel-doc:: drivers/scsi/iscsi_boot_sysfs.c
+   :export:
+
+
+iSCSI transport class
+=====================
+
+The file drivers/scsi/scsi_transport_iscsi.c defines transport
+attributes for the iSCSI class, which sends SCSI packets over TCP/IP
+connections.
+
+.. kernel-doc:: drivers/scsi/scsi_transport_iscsi.c
+   :export:
+
+
+iSCSI TCP interfaces
+====================
+
+.. kernel-doc:: drivers/scsi/iscsi_tcp.c
+   :internal:
+
+.. kernel-doc:: drivers/scsi/libiscsi_tcp.c
+   :export:
+