Indicate observed and expected sizes for "Properly size images"

**Feature request summary**

"Properly size images" is pretty good at identifying incorrectly sized images.

However, it (and [the linked documentation](https://web.dev/uses-responsive-images/)) leaves the reader in the dark on what their image sizes should actually be.

As such, it would be helpful if "Properly size images" indicated the _observed_ size and _expected_ size(s) for a given image.

Related https://github.com/GoogleChrome/lighthouse/issues/11593#issuecomment-716598218

**What is the motivation or use case for changing this?**

I'd like for my WordPress site to use correct sizes for all of its image positions. However, it's challenging to compute what the `srcset` value should actually be.

This bookmarklet is pretty helpful for determining what the `sizes` attribute should be: https://ausi.github.io/respimagelint/

Related https://github.com/ausi/respimagelint/issues/62

**How is this beneficial to Lighthouse?**

If it was easier to identify the expected image sizes, more sites would pass this audit step.