[llvm] [LangRef] State that the memory model is an axiomatic one (PR #208710)

[Ralf Jung via llvm-commits]

================
@@ -4014,6 +4014,8 @@ The LLVM IR does not define any way to start parallel threads of
 execution or to register signal handlers. Nonetheless, there are
 platform-specific ways to create them, and we define LLVM IR's behavior
 in their presence. This model is inspired by the C++ memory model.
+The memory model is defined axiomatically: it provides constraints that valid
+executions of multi-threaded LLVM IR programs must satisfy.
----------------
RalfJung wrote:

Might be good to say a bit more?
```suggestion
The memory model is defined axiomatically: we consider a set of candidate executions
where every read can (in principle) read from every write (including writes happening "later"),
and then provide constraints that reduce this candidate set to the set of actually valid executions.
```

https://github.com/llvm/llvm-project/pull/208710