iterator: add make_infix example

iterator.hpp

@@ -146,6 +146,8 @@ namespace util {
     /// the given delimiter.
     ///
     /// reduces boilerplate code required to output lists of things
+    ///
+    /// std::cout << util::make_infix (container) << '\n';
     template <typename ContainerT, typename CharT = char>
     auto
     make_infix (const ContainerT &_container, const CharT *_delimiter = ", ")