The new default values are more usable in real world scenarios
when smooth fading of distant decals is desired for performance reasons.
The Decal distance fade property hints were adjusted based on the
GeometryInstance3D visibility range fade property hints. `or_greater`
was also added to allow specifying larger values if needed.
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
