Add documentations to every public functions.

src/ftxui/dom/gauge.cpp

@@ -13,6 +13,7 @@
 
 namespace ftxui {
 
+namespace {
 // NOLINTNEXTLINE
 static const std::string charset_horizontal[11] = {
 #if defined(FTXUI_MICROSOFT_TERMINAL_FALLBACK)
@@ -158,6 +159,8 @@ class Gauge : public Node {
   Direction direction_;
 };
 
+}  // namespace ftxui
+
 /// @brief Draw a high definition progress bar progressing in specified
 /// direction.
 /// @param progress The proportion of the area to be filled. Belong to [0,1].