Fix up Pair.invert docs

- It (since a few months ago) always returns a Seq; old behaviour
    was not specced.
- Simply examples

Spec: Raku/roast@f4828aaf64
Fixes RT#131349: https://rt.perl.org/Ticket/Display.html?id=131349

Author: zoffixznet

doc/Type/Pair.pod6

@@ -197,25 +197,25 @@ of the invocant.
 
 Defined as:
 
-    method invert(Pair:D:)
+    method invert(Pair:D: --> Seq:D)
 
-Returns a new C<Pair> object with key and value exchanged, just like
-L<antipair|/type/Pair#method_antipair>, unless the value of the invocant does
-the L<Iterable|/type/Iterable> role. In the latter case an C<Iterable> structure
-of C<Pair>s is returned since C<invert> expands iterable values into multiple
-pairs.
+Returns a L<Seq>. If the C<.value> of the invocant is I<NOT> an
+L<Iterable|/type/Iterable>, the L<Seq> will contain a single L<Pair> whose
+C<.key> is the C<.value> of the invocant and whose C<.value> is the C<.key> of
+the invocant:
 
-    my Pair $p1 = (6 => 'Perl');
-    say $p1.invert;                                   # OUTPUT: «Perl => 6␤»
-    say $p1.invert.WHAT;                              # OUTPUT: «(Pair)␤»
+    :foo<bar>.invert.perl.say; # OUTPUT: «(:bar("foo"),).Seq»
 
-    my Pair $p2 = ('Perl' => (5, 6));
-    say $p2.invert;                                   # OUTPUT: «(5 => Perl 6 => Perl)␤»
-    say $p2.invert.WHAT;                              # OUTPUT: «(List)␤»
+If invocant's C<.value> I<is> an L<Iterable|/type/Iterable>, the returned L<Seq>
+will contain the same number of L<Pair>s as items in the C<.value>, with each
+of those items a C<.key> of a pair and the C<.key> of the invocant the C<.value>
+of that pair:
 
-    my Pair $p3 = ('Perl' => { cool => 'language'});
-    say $p3.invert;                                   # OUTPUT: «{cool => language => Perl}␤»
-    say $p3.invert.WHAT;                              # OUTPUT: «(Hash)␤»
+    :foo<Perl is great>.invert.perl.say;
+    # OUTPUT: «(:Perl("foo"), :is("foo"), :great("foo")).Seq»
+
+    :foo{ :42a, :72b }.invert.perl.say;
+    # OUTPUT: «((:a(42)) => "foo", (:b(72)) => "foo").Seq»
 
 =head2 method keys