KVM: nVMX: Update VMCS12_REVISION comment to state it should never change

KVM: nVMX: Update VMCS12_REVISION comment to state it should never change

Rewrite the comment above VMCS12_REVISION to unequivocally state that the
ID must never change.  KVM_{G,S}ET_NESTED_STATE have been officially
supported for some time now, i.e. changing VMCS12_REVISION would break
userspace.

Opportunistically add a blurb to the CHECK_OFFSET() comment to make it
explicitly clear that new fields are allowed, i.e. that the restriction
on the layout is all about backwards compatibility.

No functional change intended.

Cc: Jim Mattson <jmattson@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20240613190103.1054877-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>