Improve the documentation.

src/ftxui/dom/util.cpp

@@ -6,11 +6,13 @@
 
 namespace ftxui {
 
+namespace {
 Decorator compose(Decorator a, Decorator b) {
   return [a = std::move(a), b = std::move(b)](Element element) {
     return b(a(std::move(element)));
   };
 }
+}  // namespace
 
 /// @brief A decoration doing absolutely nothing.
 /// @ingroup dom