Update docs for tf.unstack with respect to numpy.

In 18692 an issue was raised over whether tf.unstack
is compatible with numpy.unstack (specified in current docs)
or numpy.split.

It looks like there is no numpy.unstack. And for numpy.split,
it is not compatible with tf.unstack.

The tf.split is very close to numpy.split. However, the second
arg `num_or_size_splits` in `tf.split` requires the number of
the splits, while the second arg `indices_or_sections` in
`numpy.split` requires the index of the splits. For that reason
the tf.split is not compatible with numpy.split as well.

According to the above this fix simply removes `The numpy equivalent` part
in the docs of tf.unstack.

This fix fixes 18692.

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