tuned comments;

--- a/src/Pure/Concurrent/unsynchronized.ML	Tue May 09 16:31:08 2023 +0200
+++ b/src/Pure/Concurrent/unsynchronized.ML	Tue May 09 16:33:30 2023 +0200
@@ -49,6 +49,8 @@
 
 (* weak references *)
 
+(*see also $ML_SOURCES/basis/Weak.sml*)
+
 type 'a weak_ref = 'a ref option ref;
 
 fun weak_ref a = Weak.weak (SOME (ref a));