Improving table matrix types comparison in tridiagonal matrix documentation.

 * memory requirements which also means better performance. See the following table

 * for the storage requirements comparison between \ref TridiagonalMatrix and \ref SparseMatrix.

 *

 *  Data types         |      SparseMatrix    | TridiagonalMatrix | Ratio

 * --------------------|----------------------|---------------------|--------

 *  float + 32-bit int | 8 bytes per element  | 4 bytes per element | 50%

 *  double + 32-bit int| 12 bytes per element | 8 bytes per element | 75%

 *  float + 64-bit int | 12 bytes per element | 4 bytes per element | 30%

 *  double + 64-bit int| 16 bytes per element | 8 bytes per element | 50%

 *  Real   | Index      |      SparseMatrix    | TridiagonalMatrix   | Ratio

 * --------|------------|----------------------|---------------------|-------

 *  float  | 32-bit int | 8 bytes per element  | 4 bytes per element | 50%

 *  double | 32-bit int | 12 bytes per element | 8 bytes per element | 75%

 *  float  | 64-bit int | 12 bytes per element | 4 bytes per element | 30%

 *  double | 64-bit int | 16 bytes per element | 8 bytes per element | 50%

 *

 * \tparam Real is a type of matrix elements.

 * \tparam Device is a device where the matrix is allocated.