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

[Justin Bogner via cfe-commits]

bogner wrote:

I think it would be helpful to add a section between the usage part and the grammar that discusses where and why in the compiler we need access to the parsed root signature (ie, in Sema for diagnostics, in the backend to verify resource usage matches, and also in the backend to lower to the serialized form). This would make it a little clearer why we need the various different representations since it will describe what we'll use them for.

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