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

[Pierre van Houtryve via cfe-commits]

================
@@ -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
----------------
Pierre-vh wrote:

Do you want me to format it like this instead to mimic the previous formatting?
```
A warning will be given if:
  - ...
```

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