[AMDGPU] Update amdgpu_cs_chain_preserve docs. NFC

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.

Differential Revision: https://reviews.llvm.org/D156408