quant docs: add and clean up ELU (#40377)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40377
Cleans up the docstring for quantized ELU and adds it to the quantization docs.
Test Plan: * build on Mac OS and inspect
Differential Revision: D22162834
Pulled By: vkuzo
fbshipit-source-id: e548fd4dc8d67db27ed19cac4dbdf2a942586759
