Mark intrinsics as safe where possible, or provide safety docs - #394

This is very helpful!

Looks like atomic operations like atomic_i_add() are still unsafe, would appreciate similar updates there.

Atomics are really difficult to make safe. You can misuse scope to get the value to an undefined state. And I'm unsure how Vulkan, let alone other APIs, handle atomic and non-atomic read/write aliasing.

All the more reasons to add "Safety" section to them. If it is difficult to rust-gpu maintainers, there is no hope for casual GPU developers like myself to use them correctly 😂

On CPU at least atomic operations on integers are perfectly safe, there is no memory unsafety that can be caused by them. I recently opened #399, maybe atomics need to start working with standard library types like AtomicU32 instead of u32 to reduce the confusion?