[llvm] [DirectX] Adding root constant documentation (PR #129569)

[Justin Bogner via llvm-commits]

bogner wrote:

While I'm generally all for smaller PRs and breaking work up into pieces, I feel like splitting up the PR for documentation like this is making it harder to review. It would be much easier to understand this if I could read the whole document at once, and I think the fact that some of the PR comments on the follow up PRs are referring to each other and to this one are evidence that others feel the same.

Do you think it makes sense to combine #129759, #129797, and #131011 into this PR so we can review the whole description of the root constants at once?

https://github.com/llvm/llvm-project/pull/129569