[llvm] [RFC][LangRef] Specify that the accessed bytes of concurrent atomics must be either disjoint or the same (PR #204329)

[Ralf Jung via llvm-commits]

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@@ -4100,6 +4105,12 @@ address. See that instruction's documentation for details.
 For a simpler introduction to the ordering constraints, see the
 {doc}`Atomics`.
 
+For the following, we call two or more accesses *perfectly overlapping*
+if they all access the exact same set of bytes, i.e., they access the
+same address and have the same access size. By the constraints of the
+previous section, racing atomic accesses must be perfectly overlapping
----------------
RalfJung wrote:

Maybe it doesn't matter too much here, since this is just a recap anyway. "act atomically" is also not defined, and if they are all reads than "acting non-atomically" is actually entirely fine.

But I think I'd find this a bit less confusing:
```suggestion
previous section, overlapping atomic accesses that are not ordered by happens-before must be perfectly overlapping
```

https://github.com/llvm/llvm-project/pull/204329