[AMDGPU] Update amdgpu_cs_chain_preserve docs. NFC

[rovka]

Summary

We no longer allow calls to functions with the amdgpu_gfx calling
convention from functions with the amdgpu_cs_chain_preserve calling
convention. See D153517.

Also mention that we can't have a chain call from
amdgpu_cs_chain_preserve using more VGPRs than it has received.

[rovka]

Rebase.

[arsenm]

llvm/docs/AMDGPUUsage.rst

1200

This sentence doesn't read right. "than the caller VGPR function parameters"

[rovka]

Fixed typo.

Does this make sense now? Usually these functions will just forward the arguments they have received, maybe with some minor processing, so the callee shouldn't receive more VGPR arguments than the caller had.

llvm/docs/AMDGPUUsage.rst

``amdgpu_cs_chain``             Similar to ``amdgpu_cs``, with differences described below.
                                      All counters (``lgkmcnt``, ``vmcnt``, ``storecnt``, etc.) are presumed in an
                                      unknown state at function entry.
 
                                      A function may have multiple exits (e.g. one chain exit and one plain ``ret void``
                                      for when the wave ends), but all ``llvm.amdgcn.cs.chain`` exits must be in
                                      uniform control flow.
 
      ``amdgpu_cs_chain_preserve``    Same as ``amdgpu_cs_chain``, but active lanes for VGPRs starting at v8 are preserved.
+                                     Calls to ``amdgpu_gfx`` functions are not allowed, and any calls to ``llvm.amdgcn.cs.chain``
+                                     must not pass more VGPR arguments than the caller's VGPR function parameters.
 
      ``amdgpu_es``                   Used for AMDPAL shader stage before geometry shader if geometry is in
                                      use. So either the domain (= tessellation evaluation) shader if
                                      tessellation is in use, or otherwise the vertex shader.
                                      ..TODO::
                                      Describe.
 
      ``amdgpu_gfx``                  Used for AMD graphics targets. Functions with this calling convention