Add documentation for List.invert

Author: lizmat

doc/Type/List.pod6

@@ -194,6 +194,20 @@ values, i.e. the direct opposite to L<pairs|/type/List#routine_pairs>.
 
     say <a b c>.antipairs;  # OUTPUT: «(a => 0 b => 1 c => 2)␤»
 
+=head2 routine invert
+
+Defined as:
+
+    method invert(List:D: --> Seq:D)
+
+Assumes every element of the List is a C<Pair>.  Returns all elements as a
+L<Seq|/type/Seq> of C<Pair>s where the keys and values have been exchanged.
+If the value of a C<Pair> is an C<Iterable>, then it will expand the values
+of that C<Iterable> into separate pairs.
+
+    my $l = List.new('a' => (2, 3), 'b' => 17);
+    say $l.invert;   # OUTPUT: «(2 => a 3 => a 17 => b)␤»
+
 =head2 routine join
 
 Defined as: