Update docstring of sparse_softmax_cross_entropy (#20242)

* Update docstring of sparse_softmax_cross_entropy

This fix tries to address the discrepancy between the docstring
and the actual implementation of sparse_softmax_cross_entropy.
The implementation of sparse_softmax_cross_entropy supports
float16, float32, and float64, though the docstring only specified
float32 and float64. This fix addresses the discrepancy.

This fix fixes 20231.

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

* Update docstring in sparse_softmax_cross_entropy_with_logits as well

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