Missing `USERDATA*` builtins in Particles shader reference

[clayjohn]

Your Godot version:
4.1
Issue description:

The Particles shader reference is missing the USERDATA* built ins which allow users to add additional data to the particles process shader

https://github.com/godotengine/godot/blob/9d089fe6e501e4818fed1b0688631c4d65d1ba35/servers/rendering/shader_types.cpp#L368-L373

URL to the documentation page:

https://docs.godotengine.org/en/latest/tutorials/shaders/shader_reference/particle_shader.html