docs: sysfs-block: document hidden sysfs entry

/sys/block/<disk>/hidden is undocumented. Document it.

Signed-off-by: Sagi Grimberg <[e-mail]>
Reviewed-by: Chaitanya Kulkarni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/ABI/stable/sysfs-block

···
 		zoned will report "none".
 
 
+What:		/sys/block/<disk>/hidden
+Date:		March 2023
+Contact:	[e-mail]
+Description:
+		[RO] the block device is hidden. it doesn’t produce events, and
+		can’t be opened from userspace or using blkdev_get*.
+		Used for the underlying components of multipath devices.
+
+
 What:		/sys/block/<disk>/stat
 Date:		February 2008
 Contact:	Jerome Marchand <[e-mail]>