[llvm] WIP: AMDGPU: Document more backend recognized attributes (PR #80239)

[Matt Arsenault via llvm-commits]

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@@ -1438,7 +1438,10 @@ The AMDGPU backend supports the following LLVM IR attributes.
                                              :ref:`amdgpu_ps <amdgpu_ps>` shaders. Any bits enabled by this value will
                                              be enabled in the final register value.
 
-    "amdgpu-wave-priority-threshold"         ..TODO:: Describe.
+    "amdgpu-wave-priority-threshold"         VALU instruction count threshold for adjusting wave priority. If exceeded,
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arsenm wrote:

ping @kosarev 

https://github.com/llvm/llvm-project/pull/80239