Added docs for List.antipairs

doc/Type/List.pod6

@@ -155,6 +155,21 @@ values.
 
     <a b c>.pairs   # (0 => 'a', 1 => 'b', 2 => 'c').Seq
 
+=head2 routine antipairs
+
+Defined as:
+
+    multi method antipairs(List:D:) returns Seq:D
+
+Usage:
+
+    LIST.antipairs
+
+Returns a L<Seq|/type/Seq> of pairs, with the values as keys and the indexes as
+values, i.e. the direct opposite to L<pairs|/type/List#routine_pairs>.
+
+    say <a b c>.antipairs   # (a => 0 b => 1 c => 2)
+
 =head2 routine join
 
 Defined as: