Categories need a description (tooltip?) when hovered/selected

[akien-mga]

Obviously the current categories are not straightforward enough to go without description, else we wouldn't have such an edit that brings an asset from the wrong category to another wrong category :P https://godotengine.org/asset-library/asset/edit/113 (the right one would likely be Materials).

[t-karcher]

I just wanted to give this one a go when I noticed that

1. there's currently no clear definition of what goes where (we have generic assets like FSMs or maze generators spread all over "scripts", "tools", "2D tools" etc.), and
2. there's already a discussion on redesigning the whole category system in #108, which should be completed first before documenting the category or tag definitions and adding links to these definitions to the front end.

So this issue is more or less dependent on / blocked by #108 at the moment.