Add FOG, RADIANCE, and IRRADIANCE builtins to Spatial shader reference

[clayjohn]

Your Godot version:
Master

Issue description:
Now that godotengine/godot#41415 is merged, we need to add entries for FOG, RADIANCE, and IRRADIANCE.

out vec4 FOG | If written to, blends final pixel color with FOG.rgb based on FOG.a
out vec4 RADIANCE | If written to, blends environment map radiance with RADIANCE.rgb based on RADIANCE.a
out vec4 IRRADIANCE | If written to, blends environment map IRRADIANCE with IRRADIANCE.rgb based on IRRADIANCE.a

URL to the documentation page:
https://docs.godotengine.org/en/latest/tutorials/shaders/shader_reference/spatial_shader.html

[MelodyMorse]

Do these go under Light Built-ins?

[clayjohn]

Nope. They go under fragment!

[skyace65]

@clayjohn you can close this issue now.