KVM: x86: Call out MSR_IA32_S_CET is not handled by XSAVES

Update the comment above is_xstate_managed_msr() to note that
MSR_IA32_S_CET isn't saved/restored by XSAVES/XRSTORS.

MSR_IA32_S_CET isn't part of CET_U/S state as the SDM states:
  The register state used by Control-Flow Enforcement Technology (CET)
  comprises the two 64-bit MSRs (IA32_U_CET and IA32_PL3_SSP) that manage
  CET when CPL = 3 (CET_U state); and the three 64-bit MSRs
  (IA32_PL0_SSP–IA32_PL2_SSP) that manage CET when CPL < 3 (CET_S state).

Opportunistically shift the snippet about the safety of loading certain
MSRs to the function comment for kvm_access_xstate_msr(), which is where
the MSRs are actually loaded into hardware.

Fixes: e44eb58334bb ("KVM: x86: Load guest FPU state when access XSAVE-managed MSRs")
Signed-off-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251028060142.29830-1-chao.gao@intel.com
[sean: shift snippet about safety to kvm_access_xstate_msr()]
Signed-off-by: Sean Christopherson <seanjc@google.com>

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