docs: another small cleanup of quantity

src/core/include/units/quantity.h

@@ -79,7 +79,7 @@ concept invoke_result_of_ =
 template<Reference auto R, RepresentationOf<R.quantity_spec.character> Rep = double>
 class quantity {
 public:
-  Rep number_;
+  Rep number_;  // needs to be public for a structural type
 
   // member types and values
   static constexpr Reference auto reference = R;