quant docs: add and clean up InstanceNorm{n}d (#40345)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40345
Fixes docstrings and adds to quantization docs for quantized InstanceNorm.
Test Plan: * build on Mac OS and inspect
Differential Revision: D22152637
Pulled By: vkuzo
fbshipit-source-id: 7a485311ead20796b7a0944827d1d04e14ec8dcd
