Add some documenation

dune/solvers/iterationsteps/mmgstep.cc

@@ -14,6 +14,7 @@ template <class MatrixType, class VectorType>
 void MonotoneMGStep<MatrixType, VectorType>::
 preprocess()
 {
+    // Unset the recompute bitfields, so we compute the full stiffness matrix hierarchy at the beginning
     for (size_t i=0; i<this->mgTransfer_.size(); i++)
         dynamic_cast<TruncatedMGTransfer<VectorType>*>(this->mgTransfer_[i])->setRecomputeBitField(nullptr);