Doc: add link to doc of sparse solver concept

Eigen/src/IterativeLinearSolvers/BasicPreconditioners.h

@@ -23,6 +23,8 @@ namespace Eigen {
   *
   * \tparam _Scalar the type of the scalar.
   *
+  * \implsparsesolverconcept
+  *
   * This preconditioner is suitable for both selfadjoint and general problems.
   * The diagonal entries are pre-inverted and stored into a dense vector.
   *
@@ -114,6 +116,8 @@ class DiagonalPreconditioner
   *
   * \tparam _Scalar the type of the scalar.
   *
+  * \implsparsesolverconcept
+  *
   * The diagonal entries are pre-inverted and stored into a dense vector.
   * 
   * \sa class LeastSquaresConjugateGradient, class DiagonalPreconditioner
@@ -172,6 +176,8 @@ class LeastSquareDiagonalPreconditioner : public DiagonalPreconditioner<_Scalar>
 /** \ingroup IterativeLinearSolvers_Module
   * \brief A naive preconditioner which approximates any matrix as the identity matrix
   *
+  * \implsparsesolverconcept
+  *
   * \sa class DiagonalPreconditioner
   */
 class IdentityPreconditioner