[llvm] [AMDGPU][Docs] Memory Model: Refactor location-order to cover reads (PR #213044)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

> A read `R` is _location-ordered_ before an access (read and/or write) `X` to the same address if it happens-before `X`.
> 
> **Happens-before is necessary for location-order.** An access `X` is _location-ordered_ before an access `Y` only if `X` happens-before `Y`.
> 
> Isn't this saying the same thing, but the latter is more general?

Yes it's saying the same thing, but in an informational section to just help the reader out. It's one of various properties that characterize the memory model.

https://github.com/llvm/llvm-project/pull/213044