Documentation: dm raid: define data_offset status field

Signed-off-by: Heinz Mauelshagen <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

Documentation/device-mapper/dm-raid.txt

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 			in RAID1/10 or wrong parity values found in RAID4/5/6.
 			This value is valid only after a "check" of the array
 			is performed.  A healthy array has a 'mismatch_cnt' of 0.
+	<data_offset>   The current data offset to the start of the user data on
+			each component device of a raid set (see the respective
+			raid parameter to support out-of-place reshaping).
+
 
 Message Interface
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