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

[Damyan Pepper via cfe-commits]

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+====================
+HLSL Root Signatures
+====================
+
+.. contents::
+   :local:
+
+Usage
+=====
+
+In HLSL, the `root signature
+<https://learn.microsoft.com/en-us/windows/win32/direct3d12/root-signatures>`_ 
+defines what types of resources are bound to the graphics pipeline. 
+
+A root signature can be specified in HLSL as a `string
+<https://learn.microsoft.com/en-us/windows/win32/direct3d12/specifying-root-signatures-in-hlsl#an-example-hlsl-root-signature>`_. 
+The string contains a collection of comma-separated clauses that describe root 
+signature constituent components. 
+
+There are two mechanisms to compile an HLSL root signature. First, it is 
+possible to attach a root signature string to a particular shader via the 
+RootSignature attribute (in the following example, using the MyRS1 entry 
+point):
+
+.. code-block:: c++
+
+    [RootSignature(MyRS1)]
+    float4 main(float4 coord : COORD) : SV_Target
+    {
+    …
+    }
+
+The compiler will create and verify the root signature blob for the shader and 
+embed it alongside the shader byte code into the shader blob. 
+
+The other mechanism is to create a standalone root signature blob, perhaps to 
+reuse it with a large set of shaders, saving space. The name of the define 
+string is specified via the usual -E argument. For example:
+
+.. code-block:: c++
----------------
damyanp wrote:

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