docs: sysfs-block: document virt_boundary_mask

/sys/block/<disk>/queue/virt_boundary_mask is completely undocumented.
Document it.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-7-ebiggers@kernel.org


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