Update docs and test cases for missing types in `tf.zeros_like` (#15934)

* Update docs for missing types in `tf.zeros_like`

This fix update docs for missing types in `tf.zeros_like`
where `half` and `string` are supported but
not listed.

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

* Also enable test cases for missing types to `tf.zeros_like`

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