Finish and polish new_game.c

Only one function is left undocumented. I tried to go into the assembly
to at least name it, but it clears fields in the save block that aren't
even documented yet.

include/main.h

@@ -68,5 +68,7 @@ void ClearPokemonCrySongs(void);
 void RestoreSerialTimer3IntrHandlers(void);
 void StartTimer1(void);
 void SeedRngAndSetTrainerId(void);
+extern u16 GetGeneratedTrainerIdLower(void);
+
 
 #endif // GUARD_MAIN_H