Revert "remove doc for sub cross because ENOSPEC (issue #165 on roast)"

Have supplied some test for 'cross' and closed #165 on roast

This reverts commit b101e5e.

doc/Type/List.pod6

@@ -895,6 +895,34 @@ Combining multiple cycles and C<:partial> also works:
 
 See L<this blog post for more elaboration on rotor|http://perl6.party/post/Perl-6-.rotor-The-King-of-List-Manipulation>.
 
+=head2 routine cross
+
+    sub cross(+@e, :&with) returns Seq:D
+
+Computes the cross-product of two or more lists or L<iterables|/type/Iterable>.
+This returns a sequence of lists where the first item in each list is an item
+from the first iterable, the second is from the second given iterable, etc.
+Every item will be paired with every other item in all the other lists.
+
+    say cross(<a b c>, <d e f>).map(*.join).join(",")
+    # ad,ae,af,bd,be,bf,cd,ce,cf
+
+The C<cross> routine has an infix synonym as well, named C<X>.
+
+    say (<a b c> X <d e f>).map(*.join).join(",")
+    # output is the same as the previous example
+
+If the optional C<with> parameter is passed, it is used as a reduction operation
+to apply to each of the cross product items.
+
+    say cross([1, 2, 3], [4, 5, 6], :with(&infix:<*>)).join(",");
+    # 4,5,6,8,10,12,12,15,18
+
+The C<X> operator can be combined with another operator as a meta-operator to perform a reduction as well:
+
+    say ([1, 2, 3] X* [4, 5, 6]).join(",")
+    # same output as the previous example
+
 =head2 routine zip
 
 Defined as: