adds documentation element

cpp/full_code/direct_numerical_simulation.hpp

@@ -55,6 +55,11 @@ class direct_numerical_simulation: public code_base
 
         virtual int read_parameters(void);
         virtual int write_checkpoint(void) = 0;
+
+        /** \brief Initialize DNS
+         *
+         * Must assign :code:`this->stat_file`.
+         */
         virtual int initialize(void) = 0;
         virtual int step(void) = 0;
         virtual int do_stats(void) = 0;