add example to crossLists deprecation message

lib/lists.nix

@@ -1692,7 +1692,7 @@ rec {
     => [ "13" "14" "23" "24" ]
     ```
 
-    To migrate to `lib.mapCartesianProduct` we need to pass an attrset as input instead of a list:
+    The following function call is equivalent to the one deprecated above:
 
     ```nix
     mapCartesianProduct (x: "${toString x.a}${toString x.b}") { a = [1 2]; b = [3 4]; }
@@ -1701,7 +1701,18 @@ rec {
     :::
   */
   crossLists = warn
-    "lib.crossLists is deprecated, use lib.mapCartesianProduct instead."
+    ''lib.crossLists is deprecated, use lib.mapCartesianProduct instead.
+
+    For example, the following function call:
+
+    nix-repl> lib.crossLists (x: y: x+y) [[1 2] [3 4]]
+    [ 4 5 5 6 ]
+
+    Can now be replaced by the following one:
+
+    nix-repl> lib.mapCartesianProduct ({x,y}: x+y) { x = [1 2]; y = [3 4]; }
+    [ 4 5 5 6 ]
+    ''
     (f: foldl (fs: args: concatMap (f: map f args) fs) [f]);
 
   /**