MAINTAINERS: add entries for block layer documentation

Include Documentation/block/ and Documentation/ABI/stable/sysfs-block in
the "BLOCK LAYER" maintainers file entry.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20211209003833.6396-9-ebiggers@kernel.org


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