[clang] [Doc][HLSL] Add documentation for root signature. (PR #83933)

[Damyan Pepper via cfe-commits]

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

There are some open comments that don't seem like they've been resolved yet (along with a couple of typos I've pointed out).

With 85 comments this PR is become quite hard to follow.  I wonder if we're at a point now where it's either good enough to go in and have issues filed against it, or if we need to abandon this one and start a fresh one?

https://github.com/llvm/llvm-project/pull/83933