[llvm] [DirectX][docs] Expand DXILArchitecture documentation (PR #84077)

[Cooper Partin via llvm-commits]

https://github.com/coopp approved this pull request.

Looks good to me.  Also good to know where these docs are located in the llvm/docs/DirectX location too.

https://github.com/llvm/llvm-project/pull/84077