block, documentation: Document discard_zeroes_data, fua, max_discard_segments and write_zeroes_max_bytes

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>