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

[Farzon Lotfi 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
+CUDA. The implementation approach for the HLSL library functionality draws from
+patterns in use by OpenCL and other Clang resource headers.
+
+Similar to OpenCL, the HLSL library functionality is implicitly declared in
+translation units without needing to include a header to provide declarations.
+In Clang this is handled by making ``hlsl.h`` an implicitly included header
+distributed as part of the Clang resource directory.
+
+Similar to OpenCL, HLSL's implicit header will explicitly declare all overloads,
+and each overload will map to a corresponding ``__builtin_*`` compiler intrinsic
----------------
farzonl wrote:

__builtin_* --> __builtin* 

seems to be how you do it elsewhere. 

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