Make a CanvasFog guide

[originalnicodr]

Since it's a not very user-friendly shader, and a lot of people are overwhelmed by the number of options, it might be worth considering making a guide on it at some point. Writing what it does, different use cases, and that sort of thing.

Roman wrote something about it in Russian, maybe we can ask him if he can/wants to write something more extensively.

No matter who ends up writing it, I would be happy to jump in and assists (if I don't end up writing it myself).