[box][fvgridgeom] improve docu

dumux/discretization/box/fvgridgeometry.hh

@@ -313,7 +313,7 @@ public:
     std::size_t periodicallyMappedDof(std::size_t dofIdx) const
     { return periodicVertexMap_.at(dofIdx); }
 
-    //! The index of the vertex / d.o.f. on the other side of the periodic boundary
+    //! Returns the map between dofs across periodic boundaries
     const std::unordered_map<std::size_t, std::size_t> periodicVertexMap() const
     { return periodicVertexMap_; }
 
@@ -504,7 +504,7 @@ public:
     std::size_t periodicallyMappedDof(std::size_t dofIdx) const
     { return periodicVertexMap_.at(dofIdx); }
 
-    //! The index of the vertex / d.o.f. on the other side of the periodic boundary
+    //! Returns the map between dofs across periodic boundaries
     const std::unordered_map<std::size_t, std::size_t> periodicVertexMap() const
     { return periodicVertexMap_; }