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/fs/exec.c b/fs/exec.c
index 8fccc27..9f9f931 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -798,7 +798,7 @@
 	if (!thread_group_leader(tsk)) {
 		leader = tsk->group_leader;
 
-		sig->notify_count = -1;
+		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
 			write_lock_irq(&tasklist_lock);
 			if (likely(leader->exit_state))