Use doxygen comment

src/catch2/catch_config.cpp

@@ -22,8 +22,7 @@ namespace Catch {
                 mutable std::ostream m_os;
 
             public:
-                // Note that the streambuf must live at least as long as this
-                // object.
+                //! The streambuf `sb` must outlive the constructed object.
                 RDBufStream( std::streambuf* sb ): m_os( sb ) {}
                 ~RDBufStream() override = default;