[clang] [C2y] Add documentation to conform to WG14 N3262; NFC (PR #98146)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Perhaps also update
> 
> https://github.com/llvm/llvm-project/blob/345861b186645f82c0b812427fdafe6ebd62a058/clang/www/c_status.html#L1283-L1285

Oh what the heck, I thought I already did that! :-D But yes, I'll update that when landing, thank you!

https://github.com/llvm/llvm-project/pull/98146