That’s because many of them are in development, so information gets old very fast. Therefore there’s no incentive to explain things that often get obsolete.
The number one rule about shaders is that you need to experience them. I mean, the quality perceived will depends on your hardware (device and display, including your output resolution) and more subtle to your subjective memory about how some retro game should be presented on a screen.
Another aspect to be aware of is that some shaders express their best quality on certain resolutions than others. And not just output resolutions (from your monitor), but source/native resolutions too (from cores).
Other than that, the Retroarch shader specs allow the user to tweak the presets easily through “shader parameter” options screen and even save them as a new preset. This implies that even the shader developers can’t precise exactly what kind of visuals their shaders can achieve!
So, for that rule above, I suggest you to try them. Just open Retroarch and begin testing all presets available until you know what each of them looks like.
Now, about your request, I suggest these two links: