Update scale and zeroPoint documentation

The description for zeroPoint appears to be empty on
https://developer.android.com/ndk/reference/struct/a-neural-networks-operand-type.html#zeropoint

Moreover, the description for the two fields has become outdated, e.g.
- TENSOR_QUANT16_SYMM only uses scale,
- TENSOR_QUANT8_SYMM_PER_CHANNEL does not use scale or zeroPoint.

Fix: 146789186
Bug: 160406237
Test: generate_api.sh
Test: m
Change-Id: I439a70405a47576da22dacf7070de3bf65ac7caf