Add method description for error fraction marking.

dune/fufem/estimators/errorfractionmarking.hh

@@ -48,6 +48,15 @@ class ErrorFractionMarkingStrategy
                          GridType& grid,
                          double fraction);
 
+        /**
+         * \brief Sorts the errors associated to all subentities of all grids
+         * and marks all those elements (for refimenent) that have a subentity
+         * among the top percentage.
+         * \param refinementIndicator For each grid, the errors associated with
+         * its entities are provided here.
+         * \param fraction Specifies the fraction of the error to be taken into
+         * account for refinement.
+         */
         static void mark(const std::vector<RefinementIndicator<GridType>* >& refinementIndicator,
                          const std::vector<GridType*>& grids,
                          double fraction);