What makes you think sync/atomic is undocumented? https://pkg.go.dev/sync/atomic
> Package atomic provides low-level atomic memory primitives useful for implementing synchronization algorithms.
> These functions require great care to be used correctly. Except for special, low-level applications, synchronization is better done with channels or the facilities of the sync package. Share memory by communicating; don't communicate by sharing memory.
There was a long mailing list discussion discussing use cases and certain desired behaviours that the people on the list agreed upon. In the end they chose not to make those behaviours explicit and leave it somewhat open-ended and instead put in the phrase "great care" to cover it off, rather than define it.
I found it while using sync.atomic to make some implementation faster, then wanted to know exactly what the language/runtime behaviour/guarantees were and was surprised but not shocked to find the 'for the go authors but not thee' attitude of the conclusion. It did have some reasonable founding where they mentioned that by formally defining it, it could be over-specified precluding future optimizations on even newer processors/architectures.
I would have preferred if the documentation explicitly listed the parts that are not fully defined with reasons for doing so given. That's better than a generic danger signpost. The irony here is that Go was supposed to be not-Java but has the same 'for mainstream' mindset rather than being the sharp tools for the sharp developers.