subgroup: fix docs and broadcast const variant

[Firestar99]

• fix broadcast docs as reported by @nazar-pc in https://github.com/Rust-GPU/rust-gpu/pull/306/files/88eb6e3b2cfb30188aa9164d529738eddeaa77dd#r2413686865
• adds subgroup_broadcast_const variant
  • prior to spv1.5 or vulkan1.2, the OpGroupNonUniformBroadcast's id param must be a constant
  • you can't actually use our subgroup_broadcast, even if the id is a constant as we'll emit it as a dynamic param instead
  • subgroup_broadcast_const accepts id as a const generic and ensures it's emitted as a constant
• Question: In the newer spirv specs they refactored the docs to respect the new "tangle" definitions, should I also update the docs of our intrinsics?

[nazar-pc]

Question: In the newer spirv specs they refactored the docs to respect the new "tangle" definitions, should I also update the docs of our intrinsics?

I'd prefer that, but when I saw it the first time, I had to search what they mean by "tangle" before I could understand what they're saying 😅 So make sure it is discoverable as well or else it becomes even less clear for newbies.

[Firestar99]

The complexity of "tangle" (despite not really adding anything substantial over "subgroup size") is my main concern as well. I mean I could sit down and write some nice module level docs on subgroups, tangle and the different operations. But I'd honestly rather spent that time documenting the more fundamental aspects of rust-gpu first.