Update particle builtins description in Spatial shader

[Calinou]

• Depends on Particles: Fix angular velocity godot#117861.

If we want to merge this now, I can remove the USERDATA1 note and split it to its own PR.

PS: USERDATA1-6 aren't documented anywhere in the manual. What would be the best place to introduce it? These are only available in particle shaders.

[paddy-exe]

PS: USERDATA1-6 aren't documented anywhere in the manual. What would be the best place to introduce it? These are only available in particle shaders.

I would argue in the particle shader page as a separate section with a barebone example on how to read/write the values.

[QbieShay]

This is incorrect. I don't know how to easily phrase it, but it contains the randomness of the lifetime.

So for example a certain particle may have 0.9 in CUSTOM.w, meaning the particle lives for 90% of the specified lifetime. So when this particle dies, CUSTOM.y will contain 0.9. Dividing Y by W gives the correct result.

I count on you all to figure out a phrasing for this, i have no idea how to make it intuitive.

[skyace65]

@Calinou Poke. Please see QbieShay's review