[llvm] [docs] Reflect C++'s read-write and write-read coherence rules in the LLVM memory model (PR #217849)

[Fabian Ritter via llvm-commits]

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@@ -4170,10 +4170,24 @@ happens-before must be perfectly overlapping to act atomically.
     read-modify-write operation M ({ref}`cmpxchg <i_cmpxchg>` and
     {ref}`atomicrmw <i_atomicrmw>`) reads from a perfectly overlapping
     `monotonic` (or stronger) write W, W must be immediately before M in
-    the relevant modification order. If one atomic read happens before
-    another perfectly overlapping atomic read and both are at least
-    `monotonic`, the later read must not see an earlier value in the
-    address's modification order. This disallows reordering of perfectly
+    the relevant modification order.
+
+    Atomic accesses that are `monotonic` (or stronger) satisfy coherence rules:
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ritter-x2a wrote:

I've added a point for write-write coherence. I think we shouldn't just state the name "write-write coherence" and that it is implied from something else without saying what "write-write coherence" means, so I gave it a proper self-contained entry in the list.

https://github.com/llvm/llvm-project/pull/217849