Issue #20006: Fix sporadic failures in test_weakset.
diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
index 990c3a6..627959b 100644
--- a/Lib/_weakrefset.py
+++ b/Lib/_weakrefset.py
@@ -60,6 +60,8 @@
             for itemref in self.data:
                 item = itemref()
                 if item is not None:
+                    # Caveat: the iterator will keep a strong reference to
+                    # `item` until it is resumed or closed.
                     yield item
 
     def __len__(self):