[llvm] [AMDGPU] Simplify and document waitcnt handling on call and return (PR #172453)

[Diana Picus via llvm-commits]

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@@ -2128,6 +2125,14 @@ The AMDGPU backend supports the following calling conventions:
 
      =============================== ==========================================================
 
+The following ABI conventions apply to all calling conventions that are used for
+callable functions (i.e. those that do not correspond to hardware entry points):
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rovka wrote:

Nit: maybe add a sentence about entry functions too?

https://github.com/llvm/llvm-project/pull/172453