Revert examples change + add doc.

examples/component/radiobox.cpp

@@ -4,13 +4,9 @@
 #include <string>  // for string, allocator, basic_string
 #include <vector>  // for vector
 
-#ifndef FTXUI_BUILD_MODULES
 #include "ftxui/component/captured_mouse.hpp"      // for ftxui
 #include "ftxui/component/component.hpp"           // for Radiobox
 #include "ftxui/component/screen_interactive.hpp"  // for ScreenInteractive
-#else
-import ftxui.component;
-#endif
 
 using namespace ftxui;