Update documentation for tabulation in vashista pair style

described above from values in the potential file.

This pair style does not support the "pair_modify"_pair_modify.html

shift, table, and tail options.

{shift} and {tail} options, but it supports the {table} option.

Turning on the {table} option will use a linear interpolation table

for force and energy of the two-body term with {2**N} entries.

Tabulation can reduce the computational cost by a factor of 2-3

with 20-bit to 12-bit table sizes, respectively, at a small to

moderate reduction in precision. It is not recommended to use

smaller than 12-bit tables.

This pair style does not write its information to "binary restart

files"_restart.html, since it is stored in potential files.  Thus, you