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

[S. Bharadwaj Yadavalli via cfe-commits]

https://github.com/bharadwajy approved this pull request.

Consider consistent spelling of the term "built in" consistently - whichever is preferred.

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