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

Completes #84839

---------

Co-authored-by: Farzon Lotfi <[email protected]>

Author: farzonl

llvm/docs/DirectX/DXILArchitecture.rst

@@ -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. The `DXILIntrinsicExpansion` pass handles all 
+the cases where our intrinsics don't have a one to one mapping. This 
+pass may also be used when the expansion is specific to DXIL to keep 
+implementation details out of CodeGen. Finally, there is an expectation 
+that we maintain vector types through this pass. Therefore, best 
+practice would be to avoid scalarization in this pass.
+
+
 The DirectX Backend
 ===================