Update the documentation of `softmax_cross_entropy`

This fix updates the documentation of `softmax_cross_entropy`,
and removed the shape restrictions of `onehot_labels` and `logits`.
They only needs to be of the same shape, not necessary `[batch_size, num_classes]`.

This fix fixes 16263.

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