Updating quantization documentation (#68907)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68907
Added information about symmetric
qschemes and corrected an error in reference to https://github.com/pytorch/pytorch/issues/68540
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D32662033
fbshipit-source-id: 9052c597f61991934b86850fea8b6eab78397450
