Clarifies and summarizes unordered classes, closes #2613

Author: JJ

doc/Language/setbagmix.pod6

@@ -4,15 +4,22 @@
 
 =SUBTITLE Unordered collections of unique and weighted objects in Perl 6
 
-You might want to collect objects in a container but you do not care
-about the order of these objects. For such cases, Perl 6 provides the
+
+In a nutshell, these classes hold, in general, unordered collections of
+objects. Sets only consider if these objects are present or not, bags can
+hold several objects of the same kind, and mixes also allow fractional (and
+negative) weights. The regular versions are immutable, the I<Hash> versions
+are mutable.
+
+Let's elaborate on that. If you want to collect objects in a container but you
+do not care about the order of these objects, Perl 6 provides the
 I<unordered> collection types L<Set|/type/Set>,
 L<SetHash|/type/SetHash>, L<Bag|/type/Bag>, L<BagHash|/type/BagHash>,
 L<Mix|/type/Mix>, and L<MixHash|/type/MixHash>. Being unordered, these
 containers can be more efficient than L<Lists|/type/List> for looking up
 elements or dealing with repeated items.
 
-If you want to get the contained objects (elements) B<without
+On the other hand, if you want to get the contained objects (elements) B<without
 duplicates> and you only care I<whether> an element is in the collection
 or not, you can use a L<Set|/type/Set> or L<SetHash|/type/SetHash>. If
 you want to get rid of duplicates but still preserve order, take a look
@@ -33,6 +40,7 @@ The types L<Mix|/type/Mix> and L<MixHash|/type/MixHash> are similar
 to L<Bag|/type/Bag> and L<BagHash|/type/BagHash>, but they also
 allow B<fractional and negative weights>.
 
+
 =begin comment
 =defn  Bag or BagHash
 Collection of distinct objects mapped to integer weights
@@ -71,7 +79,7 @@ associated weights are the 'values':
 
 =head1 Set/Bag operators
 
-There are several infixes devoted to performing common operations on
+There are several infix operators devoted to performing common operations on
 L<Set|/type/Set>, such as unions and set differences. Other operations
 include boolean checks, like whether an object is an element of a
 C<Set>, or whether one C<Set> is a subset of another C<Set>.