[clang] [clang][docs] Reflect the implementation status for P2280R4 (PR #127166)

[A. Jiang via cfe-commits]

frederick-vs-ja wrote:

> > Ideally, the C++23 behavior should also be present in C++11~20 modes. Did you want to add FIXME to these checks?
> 
> Yes, this would be an improvement.

I'd wait for @tbaederr's backport #129646. Once that lands, IIUC this PR will only contain pure documentation changes. If we even cherry-pick #129646 for LLVM 20 release, this PR would become completely unnecessary.

https://github.com/llvm/llvm-project/pull/127166