Add documentation

test/navier-stokes/howto/taylor-green.cc

@@ -254,9 +254,9 @@ void taylor_green (const GV& gv, const Dune::ParameterTree& configuration, std::
   while (time < time_end - dt_min*0.5){
     osm.apply(time,dt,xold,x);
 
-    // Correct pressure after each step. Implementation not suited for
-    // parallel computations so it's turned off for periodic boundary
-    // conditions.
+    // Correct pressure after each step. Without this pressure
+    // correction the velocity will be ok but the pressure will be
+    // shifted by a constant.
     Dune::PDELab::integrateGridFunction(pdgf,pressure_integral,2);
     pressure_integral = gv.comm().sum(pressure_integral);
     std::cout << gv.comm().rank() << " palpo pressure_integral before: " << pressure_integral << std::endl;