Improve the documentation.

include/ftxui/dom/take_any_args.hpp

@@ -1,6 +1,8 @@
 // IWYU pragma: private, include "ftxui/dom/elements.hpp"
 #include <type_traits>
 
+namespace ftxui {
+
 template <class T>
 void Merge(Elements&, T) {}
 
@@ -30,6 +32,12 @@ Elements unpack(Args... args) {
     return container(unpack(std::forward<Args>(children)...)); \
   }
 
+TAKE_ANY_ARGS(vbox)
+TAKE_ANY_ARGS(hbox)
+TAKE_ANY_ARGS(dbox)
+TAKE_ANY_ARGS(hflow)
+}  // namespace ftxui
+
 // Copyright 2020 Arthur Sonzogni. All rights reserved.
 // Use of this source code is governed by the MIT license that can be found in
 // the LICENSE file.