[llvm] [DirectX][Docs] Add DXILIntrinsicExpansion Pass to DXILArchitecture.rst (PR #86198)

[Damyan Pepper via llvm-commits]

================
@@ -61,6 +61,19 @@ on the utilities described in "Common Code" above in order to share
 logic with both the DirectX backend and with Clang's codegen of HLSL
 support as much as possible.
 
+The DirectX Intrinsic Expansion Pass
+====================================
+There are intrinsics that don't map directly to DXIL Ops. In some cases
+an intrinsic needs to be expanded to a set of LLVM IR instructions. In
+other cases an intrinsic needs modifications to the arguments or return
+values of a DXIL Op. This pass (`DXILIntrinsicExpansion.cpp`) handles all 
----------------
damyanp wrote:

```suggestion
values of a DXIL Op. The `DXILIntrinsicExpansion` pass handles all 
```

I think this is more consistent with the other pass descriptions above.

https://github.com/llvm/llvm-project/pull/86198