Document using CUSTOM* built-ins to access extra UVs

[noidexe]

Your Godot version: 4.2-beta1

Issue description:
godotengine/godot#52504 this PR added support for multiple UVs. When importing a gltf file, the first two uvs are mapped to the UV and UV2 vertex build-ins, while the rest go to the xy and zw channels of the CUSTOM* built-ins, supporting up to 8 uvs total.
This doesn't seem to be documented anywhere. The table showing vertex build-ins lists CUSTOM0,1,2,3 but they have no description. I understand they are multipurpose but I think it'd be important to state somewhere that they are automatically populated with extra UVs

URL to the documentation page (if already existing): https://docs.godotengine.org/en/latest/tutorials/shaders/shader_reference/spatial_shader.html#vertex-built-ins

[fire]

See also the proposal godotengine/godot-proposals#2266