atomic ops unsafe, but unclear why

this is basically a follow up to issue #382, where @nazar-pc said:

"""
Functions like workgroup_memory_barrier_with_group_sync() are unsafe, but it is not clear why (no "Safety" section in docs).

They should either not be unsafe or "Safety" section should be added to corresponding function docs explaining how to use them safely.
"""

Similar concern with the files in e.g. crates/spirv-std/src/arch/atomics.rs -- all of the atomic ops seem to be marked as unsafe. I don't understand the spir-v / Vulkan / whatevs memory model... like... at all... but I always thought atomic ops like these were specifically designed to provide a safe way to update memory that might be touched by multiple threads? If this is not the case, then as @nazar-pc suggested re barriers, a comment saying how to uphold any invariants required for safety would be a big help! Thanks!! :-)

Beginne mit crates/spirv-std/src/arch/atomics.rs und lies issue #382 für die zugehörige Diskussion über unsafe-Funktionen und Sicherheitsdokumentation. Ermittle, ob die atomaren Operationen unter dem SPIR-V/Vulkan-Speichermodell vom Aufrufer verwaltete Invarianten erfordern; als abgeschlossen gilt die Aufgabe entweder, wenn ihre Sicherheitsanforderungen in der Funktionsdokumentation klargestellt sind, oder wenn festgestellt wurde, dass die unsafe-Markierungen geändert werden sollten.