[docs][amdgpu] Update kernarg documentation for gfx90a (#109690)

Update the docs to mention that kernel argument preloading is not
supported on MI210.

Author: kuhar

llvm/docs/AMDGPUUsage.rst

@@ -361,7 +361,7 @@ Every processor supports every OS ABI (see :ref:`amdgpu-os`) with the following
                                                     - tgsplit           flat          - *rocm-amdhsa* - AMD Instinct MI250 Accelerator
                                                     - xnack             scratch       - *rocm-amdhsa* - AMD Instinct MI250X Accelerator
                                                     - kernarg preload - Packed
-                                                                        work-item
+                                                      (except MI210)    work-item
                                                                         IDs
 
      ``gfx90c``                  ``amdgcn``   APU   - xnack           - Absolute      - *pal-amdpal*  - Ryzen 7 4700G