Update documentation for gather_nd/gather to specify behaviors for out-of-bound indices (#15857)

* Update documentation for gather_nd/gather to specify behaviors for out-of-bound indices

This fix updates documentation for gather_nd/gather
to specify behaviors for out-of-bound indices. Basically,
on CPU an error will be returned and on GPU 0 value will be filled
to the expected positions of the output.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>