ResolutionFileResolver documented.

Whenever someone wants to use the `ResolutionFileResolver`, there's a lot of confusion. The implementation has changed over time and there is 0 documentation as to how it works in the JavaDoc. That is why I have added some docs to the class.

I have also fixed an invalid argument check (`if (descriptors == null)` became `if (descriptors.length == 0)`) and moved it to the correct place.

Furthermore, this is actually an API breaking change, because I have renamed a public field from `suffix` to `folder`, because it is not used as a suffix anymore. I think in the past it was used to search for `textures/brick.png.<suffix>`, nowadays this behaviour has changed to `textures/<folder>/brick.png`.