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

[S. Bharadwaj Yadavalli via cfe-commits]

================
@@ -114,6 +114,44 @@ not re-targetable, we want to share the Clang CodeGen implementation for HLSL
 with other GPU graphics targets like SPIR-V and possibly other GPU and even CPU
 targets.
 
+hlsl.h
+------
+
+HLSL has an extensive library of functionality. This is similar to OpenCL and
----------------
bharadwajy wrote:

```suggestion
HLSL has a library that supports extensive set of functionalities. This is similar to OpenCL and
```

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