docs: sysfs-block: document hidden sysfs entry

/sys/block/<disk>/hidden is undocumented. Document it.
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20230303084323.228098-1-sagi@grimberg.meSigned-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/stable/sysfs-block

@@ -705,6 +705,15 @@ Description:
 		zoned will report "none".
 
 
+What:		/sys/block/<disk>/hidden
+Date:		March 2023
+Contact:	linux-block@vger.kernel.org
+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 <jmarchan@redhat.com>