Update documentation for buffer fat pointers (#92034)

Now that we've got (minus some issues around datatypes and invariant
loads) working lowerings for address space 7, update the table in the
AMDGPU usage guide to properly indicate the nature of these address
spaces.

Author: krzysz00

llvm/docs/AMDGPUUsage.rst

@@ -824,8 +824,8 @@ supported for the ``amdgcn`` target.
      Constant                              4               constant    *same as global* 64      0x0000000000000000
      Private                               5               private     scratch          32      0xFFFFFFFF
      Constant 32-bit                       6               *TODO*                               0x00000000
-     Buffer Fat Pointer (experimental)     7               *TODO*
-     Buffer Resource (experimental)        8               *TODO*
+     Buffer Fat Pointer                    7               N/A         N/A              160     0
+     Buffer Resource                       8               N/A         V#               128     0x00000000000000000000000000000000
      Buffer Strided Pointer (experimental) 9               *TODO*
      Streamout Registers                   128             N/A         GS_REGS
      ===================================== =============== =========== ================ ======= ============================