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

[Justin Bogner via llvm-commits]

================
@@ -393,6 +393,161 @@ bit in the mask vector identifies one column of a patch constant input and a
 column of an output. A value of 1 means the output is impacted by the primitive
 input.
 
+Root Signature (RTS0) Part
+--------------------------
+.. _RTS0:
+
+The Root Signature data defines the shader's resource interface with Direct3D 
+12, specifying what resources the shader needs to access and how they're 
+organized and bound to the pipeline. 
+
+The RTS0 part comprises three data structures: ``RootSignatureHeader``, 
+``RootParameters`` and ``StaticSamplers``. The details of each will be described 
+in the following sections. All ``RootParameters`` will be serialized following 
+the order they were defined in the metadata representation.
+
+Root Signature Header
+~~~~~~~~~~~~~~~~~~~~~
+
+The root signature header is 24 bytes long, consisting of six 32 bit values 
+representing the version, number and offset of parameters, number and offset 
+of static samplers, and a flags field for global behaviours:
+
+.. code-block:: c
+
+   struct RootSignatureHeader {
+     uint32_t Version;
+     uint32_t NumParameters;
+     uint32_t ParametersOffset;
+     uint32_t NumStaticSamplers;
+     uint32_t StaticSamplerOffset;
+     uint32_t Flags;
+   }
+
+
+Root Parameters
+~~~~~~~~~~~~~~~
+Root parameters define how resources are bound to the shader pipeline, each 
+type having different size and fields. 
+
+Each slot of root parameters is preceded by 12 bytes, three 32 bit values, 
----------------
bogner wrote:

```suggestion
Each slot of root parameters is preceded by 12 bytes (three 32 bit values)
```

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