[llvm] [Doc][AMDGPU] Document the waitcnts required before SCOPE_SYS stores on GFX12 (PR #156424)

[Pierre van Houtryve via llvm-commits]

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@@ -15255,7 +15276,9 @@ the instruction in the code sequence that references the table.
                                                             | ``s_wait_storecnt 0x0``
                                                             | ``s_wait_loadcnt 0x0``
                                                             | ``s_wait_dscnt 0x0``
+                                                            | ``s_wait_kmcnt 0x0``
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Pierre-vh wrote:

sure

https://github.com/llvm/llvm-project/pull/156424