clarifying comment

--- a/src/Pure/Isar/class.ML	Mon Jun 18 17:50:06 2012 +0200
+++ b/src/Pure/Isar/class.ML	Sun Jun 17 20:38:12 2012 +0200
@@ -73,6 +73,10 @@
   defs: thm list,
   operations: (string * (class * (typ * term))) list
 
+  (* n.b.
+    params = logical parameters of class
+    operations = operations participating in user-space type system
+  *)
 };
 
 fun make_class_data ((consts, base_sort, base_morph, export_morph, assm_intro, of_class, axiom),