Update docstring for tf.reduce_sum

This fix tries to address the issue raised in 19498 where
the description:
```
If axis has no entries, all dimensions are reduced, and a tensor with a single element is returned.
```

does not match the current behavior. The current behavior (matches `np.sum` as well) is actually:
```
If axis is None, all dimensions are reduced, and a tensor with a single element is returned.
```

This fix fixes the discrepancy for tf.reduce_sum.

This fix fixes 19498.

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