[!313] fix documentation of SeqILU and SeqILUn

Merge branch 'fix/ilu_documentation' into 'master'

ref:core/dune-istl The current documentation of SeqILU and SeqILUn claims,
that the parameter n is the number of iterations to perform. Actually the
parameter is passed into bilu_decomposition, which states that n is the order
of the ILU decomposition. This MR adjusts the docu on SeqILU and SeqILUn.

See merge request [core/dune-istl!313]

  [core/dune-istl!313]: gitlab.dune-project.org/core/dune-istl/merge_requests/313

dune/istl/preconditioners.hh

@@ -538,7 +538,7 @@ namespace Dune {
 
        Constructor invoking ILU(n).
        \param A The matrix to operate on.
-       \param n The number of iterations to perform.
+       \param n The order of the ILU decomposition.
        \param w The relaxation factor.
        \param resort true if a resort of the computed ILU for improved performance should be done.
      */
@@ -758,7 +758,7 @@ namespace Dune {
 
        Constructor gets all parameters to operate the prec.
        \param A The matrix to operate on.
-       \param n The number of iterations to perform.
+       \param n The order of the ILU decomposition.
        \param w The relaxation factor.
      */
     SeqILUn (const M& A, int n, scalar_field_type w)