Add top-level comments for classes and descriptions for blueprints

[javagl]

Some/most of our classes (like ACesium3DTileset) do not have a class description.

If there was a description, it could appear in tooltips in some places in the editor (similar to the documentation of UPROPERTIES).

Additionally, there is a "Blueprint Description" field in the "Class Settings" of blueprints:

We might consider adding a description there as well, for the same reason.