[llvm] [HLSL][SPIRV][Docs] Document intrinsic lowering (PR #213185)

[Farzon Lotfi via llvm-commits]

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@@ -586,6 +586,61 @@ acos_f32
 represents the `acos` function from the OpenCL extended instruction set for a float32
 input.
 
+#### HLSL Intrinsics
----------------
farzonl wrote:

Instead of tracking these as HLSL intrinsics can you instead  create two sections to track things that are intrinsics that map to opengl extention and intrinsics that map to spv specific instructions? 

https://github.com/llvm/llvm-project/pull/213185