[clang] [HLSL] Document all spellable attributes (PR #213346)

[Justin Bogner via cfe-commits]

bogner wrote:

> > What do you mean here? There is no newly added file or function. What reservations do you have? I don't understand what you're trying to say.
> 
> Oh my god, I misread the pull request sequence number. My sincere apologies!

No problem. Thanks for clearing that up.

https://github.com/llvm/llvm-project/pull/213346