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.me


Signed-off-by: Jens Axboe <axboe@kernel.dk>