[llvm] [DirectX] Documenting Root Signature Binary representation (PR #131011)

[Finn Plummer via llvm-commits]

https://github.com/inbelic commented:

One general minor nit is that the motivation of each structure is not consistent in being at the start or end of the description. Personally I would place them at the start, but I would also be critical on if we need them. Some don't seem to be relevant to the binary data layout

https://github.com/llvm/llvm-project/pull/131011