Docs/admin-guide/cgroup-v2: document io.latency rotational vs non-rotational behavior

Docs/admin-guide/cgroup-v2: document io.latency rotational vs non-rotational behavior

io.latency is documented only in terms of average latency and the avg_lat
stat, which matches rotational devices.  On non-rotational devices a group
misses its target once enough of the IOs in the window individually exceed
it, and io.stat reports missed/total rather than avg_lat/win.

Describe both cases: how a miss is detected, note that the avg_lat tuning
guidance is rotational-only, and update the io.stat field list (mark
avg_lat/win as rotational-only, document missed/total).

Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>