[llvm] [DXIL][Doc] Update specification of to use TableGen list instead of dag (PR #99055)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

I have a number of tweaks and suggestions but all in all this is looking pretty good. 

I think there are a couple of interesting follow up questions that we can deal with after this goes in:
1. Can/should the `llvm_intrinsic` be optional (we'll need it to be for createHandle and stuff like that, where we're going to have to do the lowering manually)
2. Should we define an enum of our own set of types rather than using `CodeGen/ValueTypes.td`? Types like `dx.handle` and `dx.ResRet.f32` and the like are going to be annoying to represent using MVTs, and there's actually a pretty limited set of legal types in DXIL operations.

https://github.com/llvm/llvm-project/pull/99055