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

Author: Pierre-vh

clang/include/clang/Basic/AttrDocs.td

@@ -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
+create machine code that meets the request.
   }];
 }