Unchecked kernel launch should precisely state the safety contract

[torsteingrindvik]

When hovering the launch_unchecked call, no safety docs appear:

docs.rs has a bit more: https://docs.rs/cubecl/latest/cubecl/prelude/struct.KernelLauncher.html#method.launch_unchecked

But that isn't in sync with the docs on the generate module from the kernel proc macro, which only has:

   #[doc = "Launch the kernel [gelu_array()] on the given runtime"]

which we see in the image.