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

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

### Merge activity

* **Oct 2, 9:18 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.com/github/pr/llvm/llvm-project/217849).


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