[S02] be more explicit about iterating sets/bags

The intent has always been that when you use a set or bag as a list,
it behaves as a list of its keys, regardless of any underlying hash
interface it might also respond to.  You must use .pairs explicitly
to get the hash pairs out of a set or bag as a list.

S02-bits.pod

@@ -13,8 +13,8 @@ Synopsis 2: Bits and Pieces
 
     Created: 10 Aug 2004
 
-    Last Modified: 16 Sep 2010
-    Version: 225
+    Last Modified: 25 Oct 2010
+    Version: 226
 
 This document summarizes Apocalypse 2, which covers small-scale
 lexical items and typological issues.  (These Synopses also contain
@@ -1376,8 +1376,8 @@ These types do (at least) the following roles:
     Block       Callable
     Seq         Iterable
     Range       Iterable
-    Set         Associative[Bool]
-    Bag         Associative[UInt]
+    Set         Associative[Bool] Iterable
+    Bag         Associative[UInt] Iterable
     Enum        Associative
     EnumMap     Associative Positional Iterable
     Signature   
@@ -1407,6 +1407,9 @@ simultaneously, but an iterator/list itself has only one thread of
 consumption.  Every time you do C<get> on an iterator, a value
 disappears from its list.
 
+Note that C<Set> and C<Bag> iterators return only keys, not values.  You
+must explicitly use c<.pairs> to get key/value pairs.
+
 =head2 Mutable types
 
 Objects with these types have distinct C<.WHICH> values that do not change