quant docs: add and clean up hardtanh (#40341)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40341
Cleans up the hardtanh docstring and adds it to quantization docs.
Test Plan: * build and inspect on Mac OS
Differential Revision: D22152636
Pulled By: vkuzo
fbshipit-source-id: c98e635199c8be332aa6958664ff23faad834908
