Kousik Sanagavarapu [off-list ref] writes:

-/* Modify list in-place to reverse it, returning new head; list will be tail */
+/*
+ * Modify list in-place to reverse it, returning new head; list will be tail.
+ *
+ * NOTE! The reversed list is constructed using the elements of the original
+ * list, hence losing the original list.
+ */

After re-reading the original, I realize that "in-place" is good
enough clue to say that this is destructive.