making the comment more accurate

activerecord/lib/active_record/attribute_methods/read.rb

@@ -33,8 +33,11 @@ def cache_attribute?(attr_name)
         protected
 
         # We want to generate the methods via module_eval rather than
-        # define_method, because define_method is slower on dispatch and
-        # uses more memory (because it creates a closure).
+        # define_method, because define_method is slower on dispatch.
+        # Evaluating many similar methods may use more memory as the instruction
+        # sequences are duplicated and cached (in MRI).  define_method may
+        # be slower on dispatch, but if you're careful about the closure
+        # created, then define_method will consume much less memory.
         #
         # But sometimes the database might return columns with
         # characters that are not allowed in normal method names (like