docs: device-mapper: move it to the admin-guide

The DM support describes lots of aspects related to mapped
disk partitions from the userspace PoV.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/admin-guide/device-mapper/delay.rst

@@ -0,0 +1,31 @@
+========
+dm-delay
+========
+
+Device-Mapper's "delay" target delays reads and/or writes
+and maps them to different devices.
+
+Parameters::
+
+    <device> <offset> <delay> [<write_device> <write_offset> <write_delay>
+			       [<flush_device> <flush_offset> <flush_delay>]]
+
+With separate write parameters, the first set is only used for reads.
+Offsets are specified in sectors.
+Delays are specified in milliseconds.
+
+Example scripts
+===============
+
+::
+
+	#!/bin/sh
+	# Create device delaying rw operation for 500ms
+	echo "0 `blockdev --getsz $1` delay $1 0 500" | dmsetup create delayed
+
+::
+
+	#!/bin/sh
+	# Create device delaying only write operation for 500ms and
+	# splitting reads and writes to different devices $1 $2
+	echo "0 `blockdev --getsz $1` delay $1 0 0 $2 0 500" | dmsetup create delayed