[doc] Enable doxygen for cholmod.hh

So far doxygen did not process this file since
`HAVE_SUITESPARSE_CHOLMOD` was missing.
This also adds the missing editor hints.

dune/istl/cholmod.hh

 // SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
 // SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
 #pragma once
 
-#if HAVE_SUITESPARSE_CHOLMOD
+#if HAVE_SUITESPARSE_CHOLMOD || defined DOXYGEN
 
 #include <dune/common/fmatrix.hh>
 #include <dune/common/fvector.hh>