Reflow, add Introduction header, fix some typoes

doc/Language/setbagmix.pod6

@@ -4,22 +4,31 @@
 
 =SUBTITLE Unordered collections of unique and weighted objects in Perl 6
 
+=head1 Introduction
+
+The six collection classes are L<Set|/type/Set>, L<SetHash|/typ[e/SetHash>,
+L<Bag|/type/Bag>, L<BagHash|/type/BagHash>, L<Mix|/type/Mix> and
+L<MixHash:/type/MixHash>.   They all share similar semantics.
+
 In a nutshell, these classes hold, in general, unordered collections of
 objects, much like an L<object hash|hashmap#index-entry-object_hash>.
 The L<QuantHash|/type/QuantHash> role is the role that is implemented by all
 of these classes: therefore they are also referenced as C<QuantHash>es.
 
+C<Set> and C<SetHash> also implement the L<Setty|/type/Setty> role,
+C<Bag> and C<BagHash> implement the L<Baggy|/type/Baggy> role, C<Mix> and
+C<MixHash> implement the L<Mixy|/type/Mixy> role (which itself implements
+the C<Baggy> role).
+
 Sets only consider if objects in the collection 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 buti
-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
+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 these
+I<unordered> collection types.  Being unordered, these containers can be more
+efficient than L<Lists|/type/List> or L<Arrays|/type/Array> for looking up
 elements or dealing with repeated items.
 
 On the other hand, if you want to get the contained objects (elements)
@@ -49,12 +58,6 @@ Use the mutable variants L<SetHash|/type/SetHash>, L<BagHash|/type/BagHash>,
 and L<MixHash|/type/MixHash> if you want to add or remove elements after the
 container has been constructed.
 
-The six collection classes C<Set>, C<SetHash>, C<Bag>, C<BagHash>, C<Mix>,
-C<MixHash>, all share similar semantics.  C<Set> and C<SetHash> implement
-the L<Setty|/type/Setty> role, C<Bag> and C<BagHash> implement the
-L<Baggy|/type/Baggy> role, C<Mix> and C<MixHash> implement the
-L<Mixy|/type/Mixy> role (which implements the C<Baggy> role).
-
 For one thing, as far as they are concerned, identical objects refer to the
 same element – where identity is determined using the L<WHICH|/routine/WHICH>
 method (i.e. the same way that the L<===> operator checks identity). For value