[examples] Add explanation for the coupling manager

examples/liddrivencavity/main.cc

@@ -143,7 +143,8 @@ int main(int argc, char** argv)
     using MassGridGeometry = GetPropType<MassTypeTag, Properties::GridGeometry>;
     auto massGridGeometry = std::make_shared<MassGridGeometry>(leafGridView);
 
-    // We introduce the multidomain coupling manager, which will coupled the mass and the momentum problems
+    // We introduce the multidomain coupling manager, which will couple the mass and the momentum problems
+    // We can obtain the type from either the `MomentumTypeTag` or the `MassTypeTag` because they are mutually coupled with the same manager
     using CouplingManager = GetPropType<MomentumTypeTag, Properties::CouplingManager>;
     auto couplingManager = std::make_shared<CouplingManager>();