block: fix enum descriptions kernel-doc

block: fix enum descriptions kernel-doc

Fix all kernel-doc warnings in blk_types.h:

Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_READ' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_WRITE' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_FLUSH' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_DISCARD' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_SECURE_ERASE' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_APPEND' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_WRITE_ZEROES' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_OPEN' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_CLOSE' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_FINISH' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_RESET' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_ZONE_RESET_ALL' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_DRV_IN' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_DRV_OUT' not described in enum 'req_op'
Warning: include/linux/blk_types.h:371 Enum value 'REQ_OP_LAST' not described in enum 'req_op'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>