[Kernels][GPU] Document RDNA3 BF16 buffer load bug

RDNA3 has a Mojo compiler bug where vectorized BFloat16 buffer loads
return zeros instead of actual data. Test will fail on RDNA3 until fixed.

FIXME: modular#5466

Author: mcgrof

max/kernels/test/gpu/layout/BUILD.bazel

@@ -38,7 +38,7 @@ _EXTRA_CONSTRAINTS = {
     src: ["//:h100_gpu"]
     for src in _H100_TESTS
 } | {
-    "test_layout_tensor_copy_amd.mojo": ["//:amd_gpu"],
+    "test_layout_tensor_copy_amd.mojo": ["//:amd_gpu"],  # FIXME: https://github.com/modular/modular/issues/5466
     "test_layout_tensor_copy_nvidia.mojo": ["//:nvidia_gpu"],
     "test_ldmatrix.mojo": ["//:nvidia_gpu"],  # FIXME: KERN-1377
     "test_tensor_core_amd.mojo": ["//:amd_gpu"],