Correct statement re: cause of unordered iteration

My previous commit stated that `enum`s provide unordered iteration because they 
_are_ `Map`s.  That is incorrect.  `enum`s provide unordered iteration because the
relevant iteration methods _create_ a corresponding `Map` to iterate over.

This commit corrects that misstatement.

doc/Language/typesystem.pod6

@@ -791,9 +791,13 @@ the keys.
     # OUTPUT: «(E::one E::two)␤» or «(E::two E::one)␤»
 
 Note that, as the output above indicates, the iteration order of enums
-is not guaranteed.  This is because enums are C<Map>s.  If you need
-to iterate an enum in order, you may do so by sorting on its C<value>,
-for example with C<E.enums.sort(*.value)>
+is not guaranteed.  This is because the methods that "iterate over" an
+enum do not iterate I<directly> on the enum (which, after all, is not
+iterable).  Instead, these iteration methods create a C<Map> with the
+same keys and values as the enum.  Because C<Map>s provide only unordered,
+iteration the iteration methods on an enum do as well.  If you need to
+iterate an enum in order, you can sort on its C<value>, for example
+with C<E.enums.sort(*.value)>.
 
 With the use of B<()> parentheses, an enum can be defined using any
 arbitrary dynamically defined list. The list should consist of Pair