[llvm] [Doc][DirectX backend] Add documentation for root signature (PR #88795)

[Damyan Pepper via llvm-commits]

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

Some documentation is better than none.  I wonder if this would be clearer if the examples of the serialized root signature actually showed hex values rather than the psuedo-code?

https://github.com/llvm/llvm-project/pull/88795