`Material` should include brief descriptions about what each shader is used for

## How can Bevy's documentation be improved?

When looking at the docs for [Material](https://docs.rs/bevy/latest/bevy/pbr/trait.Material.html) there is no way for a user who is unfamiliar with shaders to understand what each shader is intended to be used for.

Ideally there should be a short snippet detailing what the shader does in simple terms.