[`Docs`/`quantization`] Clearer explanation on how things works under the hood. + remove outdated info
# What does this PR do?

As discussed internally with @amyeroberts , this PR makes things clearer to users on how things work under the hood for quantized models. Before this PR it was not clear to users how the other modules (non `torch.nn.Linear`) were treated under the hood when quantizing a model. 

cc @amyeroberts 
_The documentation is not available anymore as the PR was closed or merged._