message: add Csr spgemm/spgeam documentation
diff: @@ -86,6 +86,28 @@ void strategy_rebuild_helper(Csr<ValueType, IndexType> *result);
  * An additional column index array is used to identify the column of each
  * nonzero element.
  *
  + * The Csr LinOp supports different operations:
  + *
  + * ```cpp
  + * matrix::Csr *A, *B, *C; // matrices
  + * matrix::Dense *b, *x; // vectors tall-and-skinny matrices
  + * matrix::Dense *alpha, *beta; // scalars of dimension 1x1
  + * matrix::Identity *I; // identity matrix
  + *
  + * // Applying to Dense matrices computes an SpMV/SpMM product
  + * A->apply(b, x) // x = A*b
  + * A->apply(alpha, b, beta, x) // x = alpha*A*b + beta*x
  + *
  + * // Applying to Csr matrices computes a SpGEMM product of two sparse matrices
  + * A->apply(B, C) // C = A*B
  + * A->apply(alpha, B, beta, C) // C = alpha*A*B + beta*C
  + *
  + * // Applying to an Identity matrix computes a SpGEAM sparse matrix addition
  + * A->apply(alpha, I, beta, B) // B = alpha*A + beta*B
  + * ```
  + * Both the SpGEMM and SpGEAM operation require the input matrices to be sorted
  + * by column index, otherwise the algorithms will produce incorrect results.
  + *
  * @tparam ValueType precision of matrix elements
  * @tparam IndexType precision of matrix indexes
  *
  