[clang] [HLSL][docs] Document hlsl.h in the HLSL docs (PR #84081)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

Should we talk at all about why we use __attribute__ clang_builtin_alias similar to what risc-v vectors does in their docs? https://github.com/llvm/llvm-project/blob/55c466da2f2f0baa509eb709b8de8926bd498b9b/clang/include/clang/Basic/AttrDocs.td#L5425

https://github.com/llvm/llvm-project/pull/84081