srp_transport: Document sysfs attributes

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Robert Jennings <rcj@linux.vnet.ibm.com>
Acked-by: David Dillow <dillowda@ornl.gov>
Signed-off-by: Roland Dreier <roland@purestorage.com>

Documentation/ABI/stable/sysfs-transport-srp

+What:		/sys/class/srp_remote_ports/port-<h>:<n>/port_id
+Date:		June 27, 2007
+KernelVersion:	2.6.24
+Contact:	linux-scsi@vger.kernel.org
+Description:	16-byte local SRP port identifier in hexadecimal format. An
+		example: 4c:49:4e:55:58:20:56:49:4f:00:00:00:00:00:00:00.
+
+What:		/sys/class/srp_remote_ports/port-<h>:<n>/roles
+Date:		June 27, 2007
+KernelVersion:	2.6.24
+Contact:	linux-scsi@vger.kernel.org
+Description:	Role of the remote port. Either "SRP Initiator" or "SRP Target".