document de_thread() with exit_notify() connection

Add a couple of small comments, it is not easy to see what this code does.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/exit.c b/kernel/exit.c
index 413c81e..879ed6e 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -877,6 +877,7 @@
 		state = EXIT_DEAD;
 	tsk->exit_state = state;
 
+	/* mt-exec, de_thread() is waiting for us */
 	if (thread_group_leader(tsk) &&
 	    tsk->signal->notify_count < 0 &&
 	    tsk->signal->group_exit_task)