quant docs: add reduce_range explanatation to top level doc (#45305)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45305
Adds an explanatation for reduce_range to the main quantization
doc page.
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D23916669
Pulled By: vkuzo
fbshipit-source-id: ef93fb774cb15741cd92889f114f6ab76c39f051
