blk-mq: fill header with kernel-doc

Insert documentation for structs, enums and functions at header file.
Format existing and new comments at struct blk_mq_ops as
kernel-doc comments.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>