quant docs: add and clean up BatchNorm{n}d (#40346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40346
Cleans up docstrings for quantized BatchNorm and adds to quantization docs
Test Plan: * build on Mac OS and inspect
Differential Revision: D22152633
Pulled By: vkuzo
fbshipit-source-id: e0bf02194158231e0205b5b2df7f6f1ffc3c4d65
