Documentation missing Transmission Rework from #37678

[Mikagino]

Your Godot version:
4.2.1

Issue description:
In shader code TRANSMISSION was reworked to be part of SSS but that's missing in the documentation.

URL to the documentation page:
https://docs.godotengine.org/en/stable/tutorials/shaders/making_trees.html

[skyace65]

This also needs to be noted on the upgrading to Godot 4 guide. Using "Back Lighting" might also have the same effect according to Calinou in #9653.