dm raid: update Documentation about reshaping/takeover/additonal RAID types

    <#raid_devs> <metadata_dev0> <dev0> [.. <metadata_devN> <devN>]

<raid_type>:

  raid0		RAID0 striping (no resilience)

  raid1		RAID1 mirroring

  raid4		RAID4 dedicated parity disk

  raid4		RAID4 with dedicated last parity disk

  raid5_n 	RAID5 with dedicated last parity disk suporting takeover

		Same as raid4

		-Transitory layout

  raid5_la	RAID5 left asymmetric

		- rotating parity 0 with data continuation

  raid5_ra	RAID5 right asymmetric

		- rotating parity N (right-to-left) with data restart

  raid6_nc	RAID6 N continue

		- rotating parity N (right-to-left) with data continuation

  raid6_n_6	RAID6 with dedicate parity disks

		- parity and Q-syndrome on the last 2 disks;

		  laylout for takeover from/to raid4/raid5_n

  raid6_la_6	Same as "raid_la" plus dedicated last Q-syndrome disk

		- layout for takeover from raid5_la from/to raid6

  raid6_ra_6	Same as "raid5_ra" dedicated last Q-syndrome disk

		- layout for takeover from raid5_ra from/to raid6

  raid6_ls_6	Same as "raid5_ls" dedicated last Q-syndrome disk

		- layout for takeover from raid5_ls from/to raid6

  raid6_rs_6	Same as "raid5_rs" dedicated last Q-syndrome disk

		- layout for takeover from raid5_rs from/to raid6

  raid10        Various RAID10 inspired algorithms chosen by additional params

		(see raid10_format and raid10_copies below)

		- RAID10: Striped Mirrors (aka 'Striping on top of mirrors')

		- RAID1E: Integrated Adjacent Stripe Mirroring

		- RAID1E: Integrated Offset Stripe Mirroring

		Here we see layouts closely akin to 'RAID1E - Integrated

		Offset Stripe Mirroring'.

        [delta_disks <N>]

		The delta_disks option value (-251 < N < +251) triggers

		device removal (negative value) or device addition (positive

		value) to any reshape supporting raid levels 4/5/6 and 10.

		RAID levels 4/5/6 allow for addition of devices (metadata

		and data device tupel), raid10_near and raid10_offset only

		allow for device addtion. raid10_far does not support any

		reshaping at all.

		A minimum of devices have to be kept to enforce resilience,

		which is 3 devices for raid4/5 and 4 devices for raid6.

        [data_offset <sectors>]

		This option value defines the offset into each data device

		where the data starts. This is used to provide out-of-place

		reshaping space to avoid writing over data whilst

		changing the layout of stripes, hence an interruption/crash

		may happen at any time without the risk of losing data.

		E.g. when adding devices to an existing raid set during

		forward reshaping, the out-of-place space will be allocated

		at the beginning of each raid device. The kernel raid4/5/6/10

		MD personalities supporting such device addition will read the data from

		the existing first stripes (those with smaller number of stripes)

		starting at data_offset to fill up a new stripe with the larger

		number of stripes, calculate the redundancy blocks (CRC/Q-syndrome)

		and write that new stripe to offset 0. Same will be applied to all

		N-1 other new stripes. This out-of-place scheme is used to change

		the RAID type (i.e. the allocation algorithm) as well, e.g.

		changing from raid5_ls to raid5_n.

<#raid_devs>: The number of devices composing the array.

	Each device consists of two entries.  The first is the device

	containing the metadata (if any); the second is the one containing the

	data.

	data. A Maximum of 64 metadata/data device entries are supported

	up to target version 1.8.0.

	1.9.0 supports up to 253 which is enforced by the used MD kernel runtime.

	If a drive has failed or is missing at creation time, a '-' can be

	given for both the metadata and data drives for a given position.

	"recover"- Initiate/continue a recover process.

	"check"  - Initiate a check (i.e. a "scrub") of the array.

	"repair" - Initiate a repair of the array.

	"reshape"- Currently unsupported (-EINVAL).

Discard Support

1.5.2   'mismatch_cnt' is zero unless [last_]sync_action is "check".

1.6.0   Add discard support (and devices_handle_discard_safely module param).

1.7.0   Add support for MD RAID0 mappings.

1.8.0   Explictely check for compatible flags in the superblock metadata

	and reject to start the raid set if any are set by a newer

	target version, thus avoiding data corruption on a raid set

	with a reshape in progress.

1.9.0   Add support for RAID level takeover/reshape/region size

	and set size reduction.