Document usage with LinearIterationStep

Explane what it does and why one needs this

[[Imported from SVN: r4698]]

dune/solvers/norms/energynorm.hh

@@ -8,7 +8,17 @@
      *
      *  \f$\Vert u \Vert_A = (u, Au)^{1/2}\f$
      *
-     *  \todo Elaborate documentation, esp. why does a norm have a member LinearIterationStep a.s.o.
+     * As alternative to providing the EnergyNorm directly with a matrix
+     * one can also provide it with a LinearIterationStep. In this case
+     * the matrix for the linear problem associated with the LinearIterationStep
+     * is used. This is necessary because you sometimes do not
+     * know the matrix in advance. This is, for example, the case
+     * for the coarse level matrices constructed by a multilevel solver.
+     *
+     * Be carefull: This matrix is not the one representing the preconditioner
+     * induced by the LinearIterationStep.
+     *
+     *  \todo Elaborate documentation.
      */
     template<class OperatorType, class DiscFuncType>
     class EnergyNorm : public Norm<DiscFuncType>