update class documentation

dune/gmsh4/gmsh4reader.hh

@@ -19,8 +19,10 @@ namespace Dune
    *
    * \tparam Grid         Type of the grid to construct.
    * \tparam GridCreator  Policy type to control what to pass to a grid factory with
-   *                      data given from the file.
-   * \tparam SizeType     Data type for indices in the .msh file
+   *                      data given from the file. [ContinuousGridCreator]
+   * \tparam SizeType     Data type for indices in the .msh file. Should match in size
+   *                      the `data_size` information in the header of the .msh file.
+   *                      This type is used for the internal data in the caches. [std::size_t]
    **/
   template <class Grid, class GridCreator = Gmsh4::ContinuousGridCreator<Grid>, class SizeType = std::size_t>
   class Gmsh4Reader