quant docs: add and clean up LayerNorm (#40342)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40342
Cleans up the docstrings for quantized LayerNorm, and adds it to the docs.
Test Plan: * build on Mac OS and inspect
Differential Revision: D22152639
Pulled By: vkuzo
fbshipit-source-id: 38adf14b34675d1983ac4ed751938aa396e5400b
