Add a doxygen documentation.

src/ftxui/dom/flex.cpp

@@ -7,6 +7,8 @@
 
 namespace ftxui {
 
+namespace {
+
 using FlexFunction = void (*)(Requirement&);
 
 void function_flex_grow(Requirement& r) {
@@ -59,6 +61,8 @@ void function_not_flex(Requirement& r) {
   r.flex_shrink_y = 0;
 }
 
+}  // namespace
+
 class Flex : public Node {
  public:
   Flex(FlexFunction f) { f_ = f; }