blk-stat: convert struct blk_stat_callback to kernel-doc

blk-stat: convert struct blk_stat_callback to kernel-doc

Most of struct blk_stat_callback documentation is already in kernel-doc
format. Convert the remaining struct members to kernel-doc to avoid
kernel-doc warnings:

Warning: block/blk-stat.h:62 struct member 'list' not described
 in 'blk_stat_callback'
Warning: block/blk-stat.h:62 struct member 'timer_fn' not described
 in 'blk_stat_callback'
Warning: block/blk-stat.h:62 struct member 'rcu' not described
 in 'blk_stat_callback'
Warning: block/blk-stat.h:133 No description found for return value of
 'blk_stat_is_active'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>