Docs/admin-guide/cgroup-v2: note blkcg_debug_stats gates io.latency stats

Docs/admin-guide/cgroup-v2: note blkcg_debug_stats gates io.latency stats

The io.stat section says that enabling the io.latency controller exposes
the depth, avg_lat and win stats in addition to the normal ones. However,
these io.latency-specific stats are debug stats and are only emitted when
the blkcg_debug_stats module parameter is enabled, which is disabled by
default.

Make this explicit so users do not expect these fields to appear in
io.stat by default, and qualify the usage text that suggests using
avg_lat to pick an io.latency target.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>