[Cleanup] Add a warning to the code

dune/tectonic/localfriction.hh

@@ -59,7 +59,8 @@ template <size_t dimension> class LocalFriction {
                           Interval<double> &D) const {
     double const xnorm = x.two_norm();
     if (xnorm <= 0.0)
-      D[0] = D[1] = 0.0;
+      D[0] = D[1] = 0.0; // WARNING: we assume that the outer
+                         // function approaches zero superlinearly
     else
       D[0] = D[1] = func->differential(xnorm) * dotFirstNormalised(x, v);
   }