docs: cgroup-v1: Fix description for css_online

The original description refers to the comment on
cgroup_for_each_descendant_pre() for more details. However, the macro
cgroup_for_each_descendant_pre() no longer exist, we replace it with the
corresponding macro cgroup_for_each_live_descendant_pre().

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>