add a comment to bond.cpp about per-atom virial and per-atom centroid virial equivalency

src/bond.cpp

@@ -91,6 +91,7 @@ void Bond::ev_setup(int eflag, int vflag, int alloc)
 
   vflag_either = vflag;
   vflag_global = vflag % 4;
+  // per-atom virial and per-atom centroid virial are the same for bonds
   vflag_atom = vflag / 4;
 
   // reallocate per-atom arrays if necessary