Auto merge of #115577 - RalfJung:atomic-load, r=Amanieu

document when atomic loads are guaranteed read-only

Based on this [discussion in Zulip](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/Can.20.60Atomic*.3A.3Aload.60.20perform.20a.20write).

The values for x86 and x86_64 are complete guesswork on my side, and I have no clue what the values might be for other architectures. I hope we can get the right people to chime in to gather the required information. :)

I'll update Miri to respect these rules once we have more data.

Author: bors