Fix docs: clarify usage of float16 scalars in operations

Update the docs for ops:
- LOG_SOFTMAX
- LOCAL_RESPONSE_NORMALIZATION

The docs mentioned possibility to use float16 as an input type but
didn't specify that other inputs must be float16 as well in that case.

Fix: 123500722
Test: mma
Change-Id: I9028c4a109c80f0b8571fab45555818e9e4bc783