doc for id of BoundaryEntity.

[[Imported from SVN: r1253]]

grid/sgrid.hh

@@ -224,6 +224,7 @@ namespace Dune {
   public:
     SBoundaryEntity() : elem_(true) {}
 
+    //! return id of boundary segment
     int id () const { return -1; }
 
     //! return true if ghost cell was calced