Fix up documentation for new disable_[range,diagonal_matrix,permutation_matrix] fcns. * NEWS: Announce new functions disable_range, disable_diagonal_matrix, disable_permutation_matrix. * diagperm.txi: Add example showing space savings from using disable_diagonal_matrix. * ov.cc(Fdisable_range): Correct docstring which had cut-and-paste error documenting disable_permutation_matrix instead. Add seealso links to other disable_xxx functions. * ov.cc(Fdisable_diagonal_matrix, Fdisable_permutation_matrix): Add seealso links to other disable_xxx functions.

## System-wide startup file for Octave.
##
## This file should contain any commands that should be executed each
## time Octave starts for every user at this site.