pid: Improve the comment about waiting in zap_pid_ns_processes

Oleg wrote a very informative comment, but with the removal of
proc_cleanup_work it is no longer accurate.

Rewrite the comment so that it only talks about the details
that are still relevant, and hopefully is a little clearer.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>