dm: document sysfs entries

Describe attributes provided by device-mapper in /sys/block.

Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --git a/Documentation/ABI/testing/sysfs-block-dm b/Documentation/ABI/testing/sysfs-block-dm
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+What:          /sys/block/dm-<num>/dm/name
+Date:          January 2009
+KernelVersion: 2.6.29
+Contact:       dm-devel@redhat.com
+Description:   Device-mapper device name.
+               Read-only string containing mapped device name.
+Users:         util-linux, device-mapper udev rules
+
+What:          /sys/block/dm-<num>/dm/uuid
+Date:          January 2009
+KernelVersion: 2.6.29
+Contact:       dm-devel@redhat.com
+Description:   Device-mapper device UUID.
+               Read-only string containing DM-UUID or empty string
+               if DM-UUID is not set.
+Users:         util-linux, device-mapper udev rules
+
+What:          /sys/block/dm-<num>/dm/suspended
+Date:          June 2009
+KernelVersion: 2.6.31
+Contact:       dm-devel@redhat.com
+Description:   Device-mapper device suspend state.
+               Contains the value 1 while the device is suspended.
+               Otherwise it contains 0. Read-only attribute.
+Users:         util-linux, device-mapper udev rules