ktask: add documentation

hulk inclusion
category: feature
bugzilla: 13228
CVE: NA
---------------------------

Motivates and explains the ktask API for kernel clients.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Tested-by: Hongbo Yao <yaohongbo@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>