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

[Finn Plummer via llvm-commits]

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@@ -400,3 +400,100 @@ SFI0 Part
 The SFI0 part encodes a 64-bit unsigned integer bitmask of the feature flags.
 This denotes which optional features the shader requires. The flag values are
 defined in `llvm/include/llvm/BinaryFormat/DXContainerConstants.def <https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/BinaryFormat/DXContainerConstants.def>`_.
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inbelic wrote:

nit: the two lines used here and elsewhere creates a different formatting from the rest of the document

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