Fixup roundrobin docs

- It ain't a method, but a sub
    Closes rakudo/rakudo#1447
- Tweak description: lists *can* be of equal length if the number of
    items is the same

Author: zoffixznet

doc/Type/List.pod6

@@ -1159,12 +1159,12 @@ C<with> parameter with a meta-operator :
 
 Defined as:
 
-    method roundrobin(List:D: --> Seq)
+    sub roundrobin(+list-of-lists --> Seq)
 
 Builds a 'list of lists', returned as a sequence, from multiple input lists or
 other L<iterables|/type/Iterable>. C<roundrobin> returns an identical result to
-that of L<zip|/type/List#routine_zip>, except when the input lists have an
-unequal number of elements.
+that of L<zip|/type/List#routine_zip>, except when the input lists are allowed
+to have an unequal number of elements.
 
     say roundrobin <a b c>, <d e f>, <g h i>;
     # OUTPUT: «((a d g) (b e h) (c f i))␤»