Clarify that enum iteration is unordered

Because enums are Maps, they do not iterate in any defined order.
But the docs didn't mention this, and provided an example that suggested
ordered iteration.  This commit adds a note to that effect and updates the 
example

doc/Language/typesystem.pod6

@@ -788,7 +788,12 @@ the keys.
     enum E <one two>;
     my @keys = E::.values;
     say @keys.map: *.raku;
-    # OUTPUT: «(E::one E::two)␤»
+    # 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)>
 
 With the use of B<()> parentheses, an enum can be defined using any
 arbitrary dynamically defined list. The list should consist of Pair