[clang] [clang][AMDGPU] Update amdgpu_waves_per_eu attr docs (PR #74587)

[Matt Arsenault via cfe-commits]

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@@ -2659,8 +2659,9 @@ An error will be given if:
   - Specified values violate subtarget specifications;
   - Specified values are not compatible with values provided through other
     attributes;
-  - The AMDGPU target backend is unable to create machine code that can meet the
-    request.
+
+The AMDGPU target backend will emit a warning whenever it is unable to
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arsenm wrote:

Oh, I wasn't looking at the header bit here. I was also assuming this is using the sphinx formatting rules? I suppose it doesn't really matter 

https://github.com/llvm/llvm-project/pull/74587