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

[David Peixotto via cfe-commits]

dmpots wrote:

I did not see any discussion of local root signatures here. It is probably good to consider those as well when thinking about this design. There is alternate syntax for specifying both local root signatues and global root signatures starting in SM 6.3

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-state-object

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-state-object#globalrootsignature
```
GlobalRootSignature MyGlobalRootSignature =
{
    "DescriptorTable(UAV(u0)),"                     // Output texture
    "SRV(t0),"                                      // Acceleration structure
    "CBV(b0),"                                      // Scene constants
    "DescriptorTable(SRV(t1, numDescriptors = 2))"  // Static index and vertex buffers.
};
```

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-state-object#localrootsignature
```
LocalRootSignature MyLocalRootSignature = 
{
    "RootConstants(num32BitConstants = 4, b1)"  // Cube constants 
};
```

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