pid: Improve the comment about waiting in zap_pid_ns_processes

	} while (rc != -ECHILD);

	/*

	 * kernel_wait4() above can't reap the EXIT_DEAD children but we do not

	 * really care, we could reparent them to the global init. We could

	 * exit and reap ->child_reaper even if it is not the last thread in

	 * this pid_ns, free_pid(pid_allocated == 0) calls proc_cleanup_work(),

	 * pid_ns can not go away until proc_kill_sb() drops the reference.

	 * kernel_wait4() misses EXIT_DEAD children, and EXIT_ZOMBIE

	 * process whose parents processes are outside of the pid

	 * namespace.  Such processes are created with setns()+fork().

	 *

	 * But this ns can also have other tasks injected by setns()+fork().

	 * Again, ignoring the user visible semantics we do not really need

	 * to wait until they are all reaped, but they can be reparented to

	 * us and thus we need to ensure that pid->child_reaper stays valid

	 * until they all go away. See free_pid()->wake_up_process().

	 * If those EXIT_ZOMBIE processes are not reaped by their

	 * parents before their parents exit, they will be reparented

	 * to pid_ns->child_reaper.  Thus pidns->child_reaper needs to

	 * stay valid until they all go away.

	 *

	 * We rely on ignored SIGCHLD, an injected zombie must be autoreaped

	 * if reparented.

	 * The code relies on the the pid_ns->child_reaper ignoring

	 * SIGCHILD to cause those EXIT_ZOMBIE processes to be

	 * autoreaped if reparented.

	 *

	 * Semantically it is also desirable to wait for EXIT_ZOMBIE

	 * processes before allowing the child_reaper to be reaped, as

	 * that gives the invariant that when the init process of a

	 * pid namespace is reaped all of the processes in the pid

	 * namespace are gone.

	 *

	 * Once all of the other tasks are gone from the pid_namespace

	 * free_pid() will awaken this task.

	 */

	for (;;) {

		set_current_state(TASK_INTERRUPTIBLE);