[llvm] [DirectX] Documenting Root Signature Binary representation (PR #131011)

[Finn Plummer via llvm-commits]

================
@@ -400,3 +400,258 @@ 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>`_.
+
+Root Signature (RTS0) Part
+--------------------------
+.. _RTS0:
+
+The Root Signature defines the interface between the shader and the pipeline, 
+specifying which resources are bound to the shader and how they are accessed. 
+This structure serves as a contract between the application and the GPU, 
+establishing a layout for resource binding that both the shader compiler and 
+the runtime can understand.
+
+The Root Signature consists of a header followed by a collection of root parameters 
+and static samplers. The structure uses a versioned design with offset-based references 
----------------
inbelic wrote:

```suggestion
The Root Signature consists of a header followed by an array of root parameters 
and an array of static samplers. The structure uses a versioned design with offset-based references 
```
I think we should try to be as specific as possible. I assume that all root parameters are listed before the static samplers? That should be noted one way or the other

https://github.com/llvm/llvm-project/pull/131011