Add documentations to every public functions.

src/ftxui/component/component_options.cpp

src/ftxui/component/dropdown.cpp

src/ftxui/component/event.cpp

src/ftxui/component/loop.cpp

src/ftxui/component/maybe.cpp

src/ftxui/component/screen_interactive.cpp

src/ftxui/component/slider.cpp

src/ftxui/dom/blink.cpp

src/ftxui/dom/bold.cpp

src/ftxui/dom/border.cpp

src/ftxui/dom/clear_under.cpp

src/ftxui/dom/color.cpp

src/ftxui/dom/dbox.cpp

src/ftxui/dom/dim.cpp

src/ftxui/dom/flex.cpp

src/ftxui/dom/flexbox_config.cpp

src/ftxui/dom/flexbox_helper.cpp

src/ftxui/dom/frame.cpp

src/ftxui/dom/gauge.cpp

src/ftxui/dom/graph.cpp