added functionity to lib interface

src/REPLICA/prd.cpp

@@ -241,6 +241,7 @@ void PRD::command(int narg, char **arg)
   update->minimize->init();
 
   // cannot use PRD with a changing box
+  // removing this restriction would require saving/restoring box params
 
   if (domain->box_change)
     error->all(FLERR,"Cannot use PRD with a changing box");