[clang] [docs] alias attribute doc (PR #193652)

[Kartik Ohlan via cfe-commits]

Ko496-glitch wrote:

@AaronBallman  @nickdesaulniers  [gcc](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125015)- gcc confirmed the diagnostic are not implemented yet 

https://github.com/llvm/llvm-project/pull/193652