scsi: target: target_core_user.[ch]: convert comments into DOC:

Make documentation on target-supported userspace-I/O design be
usable by kernel-doc by using "DOC:". This is used in the driver-api
Documentation chapter.

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>