Clarified documentation of JacobiN method in vtkMath.

Clarified documentation of Jacobi and JacobiN method in vtkMath. Doxygen description now mentions that the output eigenvectors are arranged in columns in the output matrix and that the most positive eigenvectors are selected for consistency since the Jacobi method can calculate eigenvectors that are the negative of each other.

This is a fix for issue #15387 (closed).