Include URL to what halting problem is

For those who never heard of it.

Author: zoffixznet

doc/Type/Any.pod6

@@ -170,10 +170,11 @@ will try to descend into the element if that element implements C<Iterable>.
 
 =head2 method flat
 
-Interprets the invocant as a list, flattens it, and returns that list.  Please
-note that C<.flat> will not solve the halting problem for you.  If you flat
-an infinite list C<.flat> may return that infinite list, eating all your RAM
-in the process.
+Interprets the invocant as a list, flattens it, and returns that list.
+Please note that C<.flat> will not solve the
+L<halting problem|https://en.wikipedia.org/wiki/Halting_problem> for you.
+If you flat an infinite list C<.flat> may return that infinite list, eating
+all your RAM in the process.
 
     say ((1, 2), (3)).elems;        # 2
     say ((1, 2), (3)).flat.elems;   # 3