Clarify two comments in the set writing code and its test

Author: gpshead

Lib/test/test_marshal.py

@@ -1384,8 +1384,9 @@ def test_nan_in_set_changed(self):
         # An element which is removed from the set, and another one added,
         # while a later element is dumped (here by its __buffer__() method)
         # is still written, since it was seen; but it is then referenced
-        # only by the writer.  A NaN in that situation must not be assumed
-        # to have been remembered for later references.
+        # only by the writer, which only keeps track of objects that have
+        # several references.  The set code must cope with a NaN which is
+        # written but not tracked.
         class Item:
             def __init__(self, action, h):
                 self.action = action

Python/marshal.c

@@ -675,7 +675,9 @@ typedef struct {
     size_t order;       /* Order among the elements with the same dump. */
 } w_set_item;
 
-/* Minimal order of an element which has not been written yet. */
+/* Order of the elements which have not been written yet (plus their index
+   in the set): it sorts them after the elements written before, whose order
+   is their index in WFILE.refs. */
 #define W_SET_ITEM_NEW ((size_t)1 << (8 * SIZEOF_SIZE_T - 1))
 
 static int