Document that get_referrers can return unreachable but uncollected objects.
Fixes #505453.
diff --git a/Misc/ACKS b/Misc/ACKS
index 074042d..d8747af 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -472,6 +472,7 @@
 Rickard Westman
 Truida Wiedijk
 Gerry Wiener
+Bryce "Zooko" Wilcox-O'Hearn
 Sue Williams
 Frank Willison
 Greg V. Wilson