[clang] [HLSL][Doc] Document multi-argument resolution (PR #104474)

[Justin Bogner via cfe-commits]

https://github.com/bogner commented:

If I understand correctly from reading this, there should never be a case where both dxc and clang both choose an overload but those differ. Ie, any case where the behaviour disagrees will result in clang failing to resolve the overload because it's ambiguous. We should explicitly call that out in this document.

https://github.com/llvm/llvm-project/pull/104474