cgroup_freezer: document freezer_fork() subtleties

cgroup_subsys->fork() callback is special in that it's called outside
the usual cgroup locking and may race with on-going migration.
freezer_fork() currently doesn't consider such race condition;
however, it is still correct thanks to the fact that freeze_task() may
be called spuriously.

This is quite subtle.  Let's explain what's going on and add test to
detect racing and losing to task migration and skip freeze_task() in
such cases for documentation.

This doesn't make any behavior difference meaningful to userland.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>